"$3 a month not in order to get premium features, but to feel good," Not speaking for Trello here but keep in mind that taking money (or charging a nominal amo

"$3 a month not in order to get premium features, but to feel good," Not speaking for Trello here but keep in mind that taking money (or charging a nominal amount) creates an obligation on the part of the recipient (reciprocity principle). So if the amount to be gained is nominal in some cases people or companies would rather get nothing. There are also additional legal circumstances that taking money for a service would also possibly create that would far exceed the benefit from the revenue).