If we assume they want the project file to troubleshoot your use of their software, they will have Reaper to test and they will have the software.
If this is the case then you need only send the .RPP file.
How you find the .RPP file depends on how you have Reaper projects in a file structure and how you name them.
My structure:
I have a folder called Reaper Projects.
Inside this is a folder for each project:Blue, Red, Green, etc
Each time I finish working on a project I save it with 'save as': Blue Take 1 for the first session. Next time I finish work on Blue Take 1, I save as Blue Take 2, and so on. These will be saved as Blue Take 1.RPP and Blue Take 2.RPP.
The latest .RPP file is the one they want - the one proving difficult. The .RPP file will not contain any recorded audio. It only contains things like tracks information, MIDI information, routing and buses, and so on.
If we assume they want the complete project (unlikely I think) you would have to literally copy the whole 'Blue' folder. This would include all the audio wav files and all the information. If they do need the whole project I would clean it up first because projects can get very large.
A .RPP file is easy to send as an email attachment as it is very light - a 14GB project folder of mine has a .RPP file of just over 1mb.
I hope that is clear enough and I am not teaching you to suck eggs!...
and good luck