I intend doing a website where credit can be purchased. Almost like mobile phone airtime/units. What is the best method of doing this site? I was thinking purchase the credit and insert it into the database. Once its in the database those who request/pay for it using ecommerce will receive it by text msg or email. Is this good and secure? What are your views?

What are you trying to accomplish with this. Normally, when you pay for a service you get credits to use on the site. You can use one credit per day for advertising, for example. Every day the system will deduct a credit until there are no more credits left. That's when you disable the ad. Not sure it this is what you are trying to do.