I have had similar problems, but found a solution. It is a crazy workaround, but it works.
When I first tried to purchase something through AmiStore, it opened up Netsurf and displayed the payment website. When I clicked the button to pay with Paypal, it opened up a blank page.
I modified URL prefs to use ODYSSEY | APPDIR:ODYSSEY | "*"
https://%s*"" and disabled Netsurf.
When I tried to purchase through Amistore using Odyssey, it opened the Odyssey about page. I switched URL prefs back to use Netsurf, and clicked to purchase Radeon 2.4 in Amistore. I then highlighted and copied the address from Netsurf and closed it. I opened Odyssey and pasted the address in Odyssey. I clicked to pay with Paypal and got the message that my browser did not accept cookies. I opened up Odyssey's prefs and made sure cookies were in fact enabled. I tried to pay with Paypal again... no cookie support. So...
I once again selected Radeon 2.4 in Amistore, copied the address from Netsurf, pasted it in Odyssey, clicked to pay with paypal, copied the address from Odyssey, closed Odyssey, opened Timberwolf, pasted the address in Timberwolf and finally got to Paypal where I paid for the Radeon driver.
Whew! There has got to be an easier way of doing this!
PS: All of the above mentioned browsers are up to date.