I see there are multiple threads of people mentioning trying this over time and wanting a US based payment processor. Just wanted to add that I picked up a prepaid visa gift card and tried signing up using all the payment processors. It took a while to fill out and try each one and they all declined including the Paypal one with the prepaid visa.
US prepaid visa works on most other VR sites like BadoinkVR, VRCosplay, 18VR, POVR premium, NaughtyAmericaVR. The reason it works is the US prepaid visa/mastercards are for you US purchases only.
The only way it would work is if SLR had a payment processor that was US based that they added. They could add one of the same ones the above use to resolve it and take more orders like this.
If they don't want to, another way around this would be to use paygarden.com as a payment option. There are a few VR sites that accept it and they let you register with a store gift card like Best Buy, Walmart, etc. You can pick those up at stores or buy a digital gift card number online to then activate your account. Paygarden would be a nice additional payment option.