Can I use the paypal form to have a price and a blank for donations too?

So a buy this for $7 

with a Additional Donation Amount _______ completed by the person?

At he moment the widget is limited to an exclusive payment mode. You can only have products or donation but not both within the same widget.

I would recommend creating a link on the donation option that points to another form using the donation property of the paypal payment widget where the user can fill in their details and custom amount.

Let us know if you need further assistance

Can it be a donation but that a minimal amount be required? Like $5? Sounds silly but the paypal fees are such that having it less than that it seems a bit not worth it.

You can enforce it by setting the and inputting the minimum amount in the donation section of the Paypal widget