AT,
you should see it as an opportunity, not a problem. You were probably glad to have found your customer in the first place. The customer had a problem, you solved it. Now the customer has another problem, and again he turns to you for a solution. Congratulations, you are getting repeat business! No small feat!
What you have to learn now is how to make the customer pay for the solution for the second problem. Being polite and friendly but firm is going to help you with that.
If you handle this well, the customer will come to you when he has the next problem, and he'll already know that you're expecting to be paid again, so you'll have a lot less hassle on the third contact.
On the other hand, if you give in to unreasonable demands now, you can look forward to a lifetime of underselling yourself. Don't do it.