It looks like payments are disabled for some people at mp3stor for the time being (as well as JustMusicStore) as of 6/24/07. If the situation changes, I'll post it here.
EDIT: I attempted to make a payment from a US IP address, and then a European IP address - and it looks like if you are coming from the USA, you don't have the option of paying with credit cards.
So it looks like - at the moment - if you're coming from a USA IP address, you won't easily be able to make credit card payments on your own. On the other hand, if you have a friend that has an IP outside of the US, you might be able to get them to transfer credits to you. There is an option of "Transfer Funds" that you could use for precisely that.
MP3Stor: CC Payments Work - Outside of the US
SMS/Webmoney Options
It looks like Webmoney and SMS are my only options here (using a European IP). I didn't complete the SMS portion, but it appears that it should would ok.
What happened when you tried the SMS option?
I would suggest signing up for a Webmoney account (its free) and works pretty good.
SMS/Webmoney Options
Webmoney works fine
With Webmoney you can add any amount with a minimum of $3.00 to your account. I just did it to find out if it was easy (I had never tried to buy credits at MP3Stor with Webmoney before).
So I'm reporting back to tell you that paying with Webmoney is as simple as using PayPay - after you pick the amount at MP3Stor, you are forwarded to the Webmoney website (wmtransfer.com) and you confirm your payment. Then you're sent back to MP3Stor and your balance is automatically updated.
The only thing I'm not sure about is how easy it will be to fund your Webmoney account. If you are coming from the US, it looks like these are your best options:
1) Bank transfer. I know it might sound scary (or difficult) if you've never done it before, but it's really not. Here in Europe it's such a standard thing that it really only takes about 2 minutes online to complete one. In the US, if you have an online banking service it might be easier - you can probably do the transfer in your browser rather easily. The only downside is that most US banks tend to charge crazy fees ($10-$30 or similar) for doing a transfer unless the account is at the same bank. Maybe you will be lucky and a transfer will be easy.
2) Get a CashU card (or if you're in the UK or Europe, check UKash or paysafecard). You can fill this card with a credit card and then transfer it to your Webmoney account here. This has the advantage that you can use your credit card, but you've got to go through an extra step. I signed up there, but I didn't add money to my balance - but I found that you can use your credit card if you are coming from Europe or the United States (and pretty much anywhere else).
3) Transfer from a different e-currency. There are several websites that will exchange money from PayPal into Webmoney (for example, here here and here, but I'd probably try the CashU (#2) method first. Those exchange sites charge you between 13.9-17% to make the exchange using PayPal.
I hope this is helpful!
Webmoney
Mp3Stor.com Payments
payment to mp3stor
Updated
Credit cards from the US did
I wonder why cc payments