This is what Mullvad says on their payment page.

Following these instructions helps us to process your payment more quickly

Do not use the same token more than once. Do not send coins, checks, or money orders. Do not send parcels or boxes. Do not seal the envelope with excessive amounts of tape. Do not use registered mail. Do not send large amounts of cash. Expect post to take at least two weeks before it arrives in our PO box. We check our PO box and credit accounts every day of the working week. Please be patient if your account has not been credited. We deal with payments as quickly as we can.

I guess they aren't saying that they won't accept registered mail, just that it will take longer to process.