|No such thing as a small change|
The client is a startup company, so it doesn't have a huge bunch of capital to spend on the site. They also consider advertising to be the one thing to spend most of their money on, not the development of the site. Obviously the site is important to them, or they wont spend such a lot of their startup capital on it. The site will basically be their means of income, so it is a crucial part of the company.
Company decided to be cheap on basic mean of income, and instead to spent most of money on marketing? Looks like "get-rich-quick" scheme to me.
If you decide to go for it, at least get agreement to be paid step-by-step. Protect yourself before possibitity that company will go belly up before you got paid.
For me is very hard to imagine a situation where I can use database without solid atomic transactions and rollback. If MySQL cannot support rollback, I will avoid using it for important production data even if paid to do so. And in no case for payment & Credit Card processing. Are you looking for troubles?
You are taking rather big risk, I hope at least your award is worth it. :)