3 options. 2 of them require them to pay you more.
1. You tape them up, they paint around them. Their problem. You warned them to paint before.
2. Put up a vinyl stencil the same or close to the color of the lettering for an additional fee and have them pay for the lettering now. When they paint the wall you come back and install the lettering by aligning it with the painted over stencil and they pay you for the installation.
3. Pin mount the lettering - Plot a paper pattern and secure the lettering to the paper. Carefully flip it over and use your finger to find and crease the back of the paper pattern. From the back drill a hole the size of the smallest finishing nails or wire brads you can find then remove the letters from the pattern and glue in the nails. Make sure you don't drill too deep, half way should do it. when depth is critical I will wrap a piece of tape around the bit as a depth gauge. When installing I would pre drill the holes in the sheet rock or use a nail to make a hole first. That way you won't risk pushing out the front of the letter driving it in. Charge for your time. If you do option 1 the painter sure as heck will.
You may be able to heat the nails an melt them in the pvc as well. Test first. I haven't tried it, just a thought I will try some day.