I wouldn't be concerned about the give aways to the customers. Those shouldn't be issues under section 8.
While there's always the risk or criticism on the issue, I think you could be ok with the $300 per month as long as you can justify that it is a shared cost of the advertisements. You probably need to determine what the overall cost is to see if your share is reasonable. If the builder is only paying $300, and charging you $300, you have a problem.
This is addressed in HUD's RESPA FAQs
. Question 18 states:
18. Can a mortgage banker and a real estate broker advertise their services together, for example, on the same brochure or newspaper advertisement?
A. Nothing in RESPA prevents joint advertising. However, if one party is paying less than a pro-rata share for the brochure or advertisement, there could be a RESPA violation.