It seems like it's just money and the mentor picks the gift, except the fact it's District 11 bread implies that they can pick the gift. I guess the simpler version is that they send a request it be their bread so Rue would know. I would assume it's possible to send weapons but just not worth the cost when there are weapons everywhere.

Probably you can send anything, though personally I'd think a set selection would be better so people don't try to find loopholes.