Since you're using a custom shipping and payment modules, have you tested using any of the defaults?
Does the same thing happen then?

What else is installed / custom? How can we recreate this, what are your checkout steps (guest, login, register etc)?

I've just tested it in a fresh install of Arastta 1.6.1 with default settings, only installed and used my Pay On Pickup module, and found no issues.

Note! I branched your reply into a new topic, because I hardly see them as directly related.

All the modules I use came from the marketplace.
I don't allow guest checkout.
This is going through the purchase using an existing account.
I'll check again this weekend and see if it continues.

Pay in store is a custom module made by you, nomater if it's in the Marketplace or not.
Also, Marketplace is all custom extensions in some way. You should test with the default core modules when you see any issues, to verify if it's a core issue, or a custom extension issue ...

You say you're using an existing account, but it's not complete in the image you posted, as it's missing address data, thus I'm asking for the steps to recreate it.

The pay in store is just an existing module from the marketplace with the names changed. It's from the existing 'free checkout'.

Yes using an existing account. Image was before I completed the sale.
If I continued through the sale, and then generate the invoice and print it, it shows the current sale items and the previous sale items.
I didn't notice it until a customer pointed out that something was on the invoice that they didn't order, but was actually the item from the previous sale.

Thankfully the customer caught it before they paid and left.

So it was an existing, but not complete, account - where address data was missing.
The problem is that your system doesn't clear the cart on completed orders.
Why? That part you have to figure out since it hasn't been possible to recreate it on a fresh install.

Yes it's an existing account that was setup a couple months ago.
It's setup as:

First name: Cash
Last name: Sale
With our store address.

I use it for walk in sales, phone sales, etc.
For my use for sales at the store.

I just attempted to do another sale and added quantity 3 of a product.
Cart only shows the 3 of the same product in the cart.
Go through the checkout and I login in, select the appropriate address.
The shipping modules update the prices, I select pick up in store and the cart still look fine showing only the 3 items I added.
Go to the next step, where I am to choose a payment format, and presto..... Another item previously appears in the cart.
This item was previously in the cart at another visit of a sale I didn't complete.

So this is supposed to happen from what your telling me, and that's fine.

So the problem now would be that the popup cart view top right isn't updating properly.

When I went on the website, and before I added anything to the cart, the top right cart icon next to the store banner showed "0 Item(s) - $0.00".
So like you say and it's retaining the items in the cart from the previous visit, then the top right corner cart info is incorrect.

Here's a screenshot to show the second item of "Formic Acid" was in the cart from the previous visit and only appears when selecting the payment.
The top right corner doesn't show it. Like you mentioned, refresh the page and then it appears.

So I adjusted the order to have only what I wanted in it and completed the sale.
I closed the browser, and then opened it again, and when I went to the store again,
the 3 products still appear in the cart after the sale was completed.

So the cart doesn't appear to be clearing after a completed sale.

More investigating to come.

Something that is weird, if you look at the screenshot of the completed sale, the top right corner shows 0 items and $0.00.
Close the browser and open again to visit the website, and the second screenshot shows the items in the cart again.

So I tested it again using Firefox instead of Chrome, and did a complete sale.
Everything went fine and closing the browser and opening again, and the cart was empty.

Seems that the store isn't working well with Google Chrome and that seems so far to be where the issue is.
As much as I like Chrome, it has many faults with caching.

Now you just need to ensure it's not your Chrome configuration, or any installed Chrome extensions.

So here's an update.

I have tried it in 3 different cities over the weekend on 3 different computers using Chrome, and the tests all performed the same.
Complete a sale, close the browser, open the browser and return to the website. Do another sale and the items from the old sale appear in the cart still.

On one of the computers at my dad's, Chrome was a completely new install.

I guess it's Chrome and the store not playing nice together.

Chrome browser issue confirmed: https://github.com/arastta/arastta/issues/707

PS! To avoid it in meantime you can click on the continue button after placing the order, before closing the browser, if you like to use Chrome.

It did it also on windows 10 Explorer today if anyone is interested.

Internet Explorer or Edge?
Which version?

Breck Demers wrote:
... if anyone is interested.

- unneeded and useless comment addition.

Your correct. It was Windows 10 Edge and not Explorer.

Just another update as I looked into this again as a customer complained that it's happening to her.
She placed her last order July 5/17. Today Aug 23/17 she attempted to place an order and the last order of July 5/17 appeared in her cart after she added a new item to the cart.

I did test purchases on my android, and then went to my computer to attempt a test transaction, and when I added the item, and then refreshed the page, the previous items from the previous order I made on my android appeared in my cart on the computer. So not device related issue but more likely website side. I went into the store settings, and "Cleared the Cache", and then repeated test transactions on the android and on the computer over and over and I couldn't get the issue to happen again.

I am wondering because there is a setting in the store settings to keep cache "Lifetime 86400 secs".
Is there a way to confirm the cache is being cleared every 24 hrs?
I have "Storage = File", is there a better setting to use to test?