Add to cart button disappeared?

Yes, the amount was previously rounded by HikaShop. Now, the payment plugin has to round it himself if necessary.
In our payment plugins, we were already rounding the amount even in previous versions of HikaShop so that's why it still works for them.

Anyway, your solution is fine so you're good to go.

I got an answer from webtopay and they said that they'll look at fixing the problem.