IME, merchants can restrict the payment methods available. I can't immediately point to any examples I've come across but seem to remember that historically, many merchants didn't have AMEX as an option via PayPal but the other methods including MC & Visa were available. I think more recently, AMEX (and I believe Diners) have been commonly available.I know in the past paypal does allow the companies to restrict payment methods, and I just got told that again as i called Paypal support. Paypal level 1 support on the phone reckon there is not restrictions or limitations being applied on my account and it must be an issue with the seller but I am still going to dig into Paypal more as NRMA insists they are not restricting any of the payment methods. So maybe its some imbedded issues with my paypal account that only started recently. As I was able to pay my parents home insurance with nrma via paypal and Amex only back in late jan. So if NRMA insist they havent changed anything its some odd issue with my account but yet sydney water is showing all methods. I reckon though paypal must have different options and perhaps some option that nrma use for paypal payments is not working with my account right now.
I hope I'm wrong and it's just an issue with your account @lonewolf please let us know when you've found out definitively.