Although, as being late to the party, here are my 0.02 £:
If you want to promote Retropie as project best is (IMHO) to wear a retro shirt of a popular (or your favorite) console/arcade game or brand. To this people can relate and it's easier to engage them in a conversation in which you can interweave the benefits of Retropie.
My personal experience is whenever I am on a social event with my Pac Man / Xevious / C64 / ... Shirt, people start asking me about my shirt.
To actually support the Retropie Project, either: spend them money (Patreon, Bitcoin, Paypal, aso.) (good), do testing and bugfixing, documentation (better), foster the codebase by feature extensions (best).
At the end of the day that's why the Raspberry was invented: To empower yourself by learning and doing stuff.
Plus: Preparing and merchandising Retropie specific stuff would use precious hours of the project owners, which -as a reminder- do the Retropie project as a hobby.
HTH
