The figures do add up so it makes sense in that respect, but the subtotal shouldn’t include tax. I can see how the various options make their differences, but I guess the Checkout API itself needs updating.
Update: okay, so I’ve read that various other payment platforms do include tax in the subtotal, but for many situations it makes much more sense to not have tax included in the subtotal. Having this as an option would be a good thing!
Surely though, with reference to Square Checkout pages such as shown in the screenshot below, we at least need it to state something like "Including tax" or (tax incl) above the line items prices on the right, and "Excluding tax" or (tax excl) above the line item prices on the left…
Because as it is now, the result of the multiplication of the item price by its quantity is the wrong answer, but it wouldn’t be if they were labelled accordingly.
Here’s that screenshot, which I also posted in my opening post…
Update: for anyone interested, I show how I’ve injected my PHP session variables for when using Order instead of QuickPay, in post 9 here: HTTPS via curl_init() versus HTTPS via -X POST - Questions - Square Developer Forums