If you have problems of rounding between Prestashop and PayPal, you can try this configuration on your Prestashop and make a payment test with the same kind of order, ie a product with several quantities. => Go to preferences => General => Rounding rule: Round to infinity when the value is halfway (recommended) => Rounding Type: Rounding for each item
The problem is that PayPal and PrestaShop do not calculate rounding in the same way, but normally by configuring your PrestaShop as this should correct. The rounding must be for each item, because PayPal accepts prices with only 2 digits after the commas. As long as in the prestashop, we can have the prices with up to 6 digits after the commas. For PayPal to accept payment, it is necessary to round the price of each item.