In this panel the character says:
ありがとう バッグを完成させてくれて
The bag in question was designed and made not by him, he just made requests about its characteristics, and the character he is speaking to designed and made it; this is kinda confusing for me: given the context, I think the sentence should mean something like "Thanks for finishing this bag [for me]", but させる means "to make/let do something" and くれる adds a meaning of "for me/as a favor to me".
This page agrees that させてくれる "is used when you are the actor, and you're grateful that the causer has allowed you to do the action", and the example agrees with "[someone] let me do [something]"; given this, that sentence seems to mean something like "Thanks for letting me completing this bag".
As full context: A did the bag, B (the speaking character) said the he would use it in his entire collection if A was able to modify it according to B's requests, and A did.
So I have two guesses:
It's a (over?) polite way to say "Thanks for doing this for me".
It means "Thanks for having completed it according to my requests" (so "for letting me completing it" meaning "you accepted my requests, so in a sense you let me completing it").
I think it could be the latter, but I'm not sure.
