@ the OP/anybody else interested.
Have you tried purchasing credits via the mobile phone option?
As of today's exchange rates:
To purchase wgt credits via a credit card (if US based) the return for $20 worth = 2,000 wgt credits.
$19.99 package - if purchased through Zong - will return almost 3,000 wgt credits if US based.
Buying through Zong if UK based will return 1922 wgt credits for £10.
I don't buy anywhere near as regular now as I once did. If I really need to I stick £10 on an old pay-as-you-go mobile. My sim card provider then give me my monthly allowance of texts/call minutes/data (more than enough for me) and I just use the deposited £10 to purchase credits on here.
All you do is insert your mobile number after choosing the package you want. You'll receive a 4 digit code from Zong to confirm the purchase. As soon as you reply you'll get another text confirming the transaction.
Takes seconds to do, with no extra charges whatsoever. I've never ever had a problem doing so.
Hope the above helps.