Do you think those low wage workers in the local store make marketing decisions? At best, you might make them aware of what the company they work for supports. Those decisions are made many steps over their heads. But at least you feel better.
The OP is right. Boycott Walgreens.
The low-paid guy who takes the call can pass it up the line to the high-paid guy who makes the decisions. Or Walgreens can just read their mail.
I would think it would be obvious to speak to the Store manager, or find an online source to go higher up the chain.