When a client/customer makes a special request you might be tempted to say “no”. You have too much work, you don’t have enough time etc. If you do that, you are making the final decision.
On the other hand, why not allow the client to make the decision? Maybe you can offer to do the special request but let them know there is a cost to doing so. If the special request is that it’s a rush job, why not consider adding on a significant (but reasonable) rush fee? If the special request is something that is more complicated (but doable) why not consider an “expertise fee”?
If you make this offer and the client accepts…great! If you make this offer and the client turns you down…that is the same result as if you said no in the beginning.
When possible, let the client make the decision.
Have a great day!