It's pretty simple. A guy can either purchase credits and then contribute the credits to you or he can do a direct contribution where he just uses his credit card to fund you in one-off transaction. It makes no difference on your end, a credit is a credit.
BUT, it makes sense for you to enlighten guys about the benefits of premium membership.
When a guy buys 75 or more credits in a single transaction, he automatically becomes a premium member for life. No fees, no recurring charges, no re-ups. A single purchase of 75 credits gets him:
Discounted credit pricing
Higher spending limits
2-way video conferencing
The ability to send private photos to the girls
The ability to initiate live chat with the girls
So there's a lot of benefits for you when a guy goes premium -- credits are cheaper for him, so he can afford more for you; he can cam to cam; and has more freedom to interact.