If you test a lot
First of all, you should have your billing info (address, zip, VAT (for companies)) set at the Billing page, and payment information added at the Payment info pop-up:
At the same Billing page, simply press 'Buy more' button at the 'Remaining API tests' section and get one of pop-ups:
For users that must pay VAT tax:
And for other users:
Select the desirable Credit pack and proceed.
Notice: extra credits do not expire, they start to be used only after you use all your 'plan' credits (given monthly).