Hello,
I did not solve the problem, but I managed to investigate it further. I took a copy of my shop, and
set it up as a test. I also set up PayPal sandbox.
I have only 2 digits after the dot (23.05 Euro). However I have also a Catalog Price Rule of 10% discount,
and this seems to cause these problems.
Prestashop calculates the total for a product by applying the discount to the full amount.
For example if I have 10 products, each costing 10.09, then the total is (10 * 10.09 * 90% = 90.81)
However, in the list of products, Prestashop shows the price (10.09*90% = 9.08). It seems that
it sends this price to PayPal, together with the quantity 10. So of course the two amounts will not match.
I did not have this problem before installing 1.6.1.7. This seems a bug. This now happens in my shop
quite often, and customers think that they made a mistake, they try again, and they are charged twice.
The products also stay available.
Another annoying problem, when using the PayPal sandbox is that after a payment, prestashop
shows status "Awaiting PayPal payment" instead of "Payment accepted", or the error for
the payment amount.