Tip your Porter

[tristantio]

I get this definition for donation btw:

do·na·tion   [doh-ney-shuhn]
noun
1. an act or instance of presenting something as a gift, grant, or contribution.
2. a gift, as to a fund; contribution.

I do not see where it mentions voluntary, however, looking further into being literal, look at "voluntary":

vol·un·tar·y   [vol-uhn-ter-ee]
adjective
1.done, made, brought about, undertaken, etc., of one's own accord or by free choice: a voluntary contribution.
2.of, pertaining to, or acting in accord with the will: voluntary cooperation.

A voluntary donation does not necessarily mean optional, it means done by your own choice. If you choose not to, you do not meet the requirement of "porting for donations".