Hi Nicolas,
Following this thread as we also have an issue with Paypal not applying the GST, Paypal says there is a mismatch coming from too many decimals after the decimal point. I have set the International fractional digits and local fractional digits to 2. This doesn't happen often but it does happen. When it does happen, we get a successful payment notification from Paypal, but the GST is not shown, nor is it charged to the recipient. We then receive a notification from Hikashop that says Paypal payment notification refused: Invalid amount. Message further states that "A Paypal notification was refused because the amount received (91.46AUD) is different from the order amount (100.6AUD).
I note that you fixed something in the back-end of the original poster, but didn't say what you fixed and as this issue seems to be similar to this thread, I thought I would post here. All the best, Steve