Marcia,
The problem is most people, and with their browser settings, have their cookies removed daily. A cookie may work forever but it won’t matter much if the cookie is removed when a user clears their cookies out of their browser.
In fact, I manually clear my cache and cookies a few times daily and at the very least, automatically every night and I’m sure many other people do to.
Most people will be very attracted to that free gas promotion and will likely fill out the information. Depending on when they check their email, they may have cleared their cookies or will check their email later the next day.
I think it would be much better if the affiliate tag was somehow connected and stored along with their email address when they fill it out. This way the affiliate tag will arrive with the email and the affiliate will always get credit.
If the visitors clear their cookies, there is that extra potential that the affiliate will not get credit for their referrals. And yes, that free gas promotion is probably very attractive and a high percentage of visitors will probably fill it out so it increases that potential for a loss.