As we are on this topic, this mainly only goes to shop owners really, Because we can have a normal member that just likes to give back to the community, but isn't very well known/doesn't have a huge business, so doesn't get a lot of pm's/important pm's per day, for this we could have 1 user group rank that could be "Business Owner" as a new rank, for people that would like to donate for more space in the pm, and maybe different perks. This would make people want to donate more, and also make people more trustworthy to that person, I would put some pre-req's on it first though, like must have a 1k+ post count and 100+ feedback or 200+ feedback alone, etc.. If you get what I'm saying, this would make it much easier to implement and I think would be a good addition.
Edit: I wouldn't set this at like $5, $25 or maybe even $50, maybe $75+, because this would be very trustworthy.