If they accept, you can use any of the Visa or Mastercard gift cards they sell at CVS and other stores using the brands Vanilla or Green Dot. There may be some others from American Express. I don't think you need to register unless you want to reload them.
They do come with fees which could be in the $3 to $5 for $50 or $100

is if the purchase is an online international or recurring transaction.

If P411 has structured things in such a way as to bill domestically within the US, and the transactions are non-recurring... then any VISA gift card that can be registered online will work.  

Note that registering a card online is simple. All that matters is that the name and address registered online matches the name and address provided to the merchant.

Also, a prepaid credit card is vastly different from a prepaid gift card... it is in NO WAY anonymous, although it does work for international and recurring purchases.

Zen is right about there being a huge difference between a gift card and a prepaid debit card.  

It's my experience that prepaid gift cards only need to be registered with a zip code to use online but I haven't been able to get any of the various brands to work for p411.

A prepaid debit card should work (works for some I know) but you have to apply for them with your real name, DOB, SSN, etc.

Do you have a regular friend that's a member that would be willing to use her card for you if you gave her the cash?  Could be the easiest solution if you know someone.

Good luck!

And inquire which specific prepaid gift cards will work with their site, as there are quite a few that will not.

Do not use any Green dot cards with them. GD themselves red flag accounts that are used on adult web sites. 😞

Just wanted to update on the prepaid cards for P411.  Did not have any trouble using a prepaid visa card purchased at Target.  You just have to update the account through chat on the web.  Paid with cash and had not problems.