The error message is quite misleading... says a membership _may_ increase your limit, but then it turns out, no it doesn't.
> You have exceeded the limit of 1 personal messages per hour. Buying a Copper membership may increase your limit.
- Some of the same permissions as Member-rank members, such as reduced signature styling restrictions. (But none of the PM-related restrictions are currently lifted, such as the style limit or per-hour PM limit.)