Ultimately, every position you can possibly advocate - including the position that you want people to purchase something or donate to someone - has packaged with it some vision of how society might best operate. In other words: in a society where decisions are made in a political way (which is a good thing), every message is political. Every message.
Fine:
Donate to fight cancer
Not fine:
Donate to Candidate X to support cancer research
I also donate to ACLU, but you have to be willfully obtuse to not accept that it predominately backs left-leaning policy viewpoints, and vocally so — again, fundamentally political in nature.
PP can as well. They can't say "vote pro-choice" but can advertise that they provide women's health services including abortions.
Basically: you can talk about what you do that isn't related to elections or votes in congress.
So in your example, I think it's absolutely clear to most adult Americans what "Donate now" means. It means "Donate now so that we can fight for your gun rights" or "Donate now so that we can fight for a free and open internet".
I do worry that Twitter is just ratcheting up the difficulty they're already facing with political speech on the platform.