Why was there no acknowledgement of my tip-?

I had two resin model kit statues assembled and painted by two different professional model builder/painters. In each case, I added 50 bucks to show my appreciation for the work they did.

Our communication was through emails only. What makes me wonder is that neither of them had anything to say about the additional fee. What could their reasons be-?

This is the first time both painters had me as a client and I’m quite certain the rates they gave me were quite fair.