If I buy something worth 17$ from an SMB's website, and the SMB buys their ingredients from their merchants via stripe for 10$, then those suppliers get a bill for 8$, then that's still only 17$ of GDP, but 35$ of stripe transactions.

You’re absolutely right i didnt think of that. Isn’t this what they call velocity of money? So we’d need to calculate the “velocity of stripe” (the flow of dollars within the system).

I think the parent is correct in saying that’s an economic activity though it’s not GDP (Gross Product). It’s difficult to do circular transactions with no “economic value” in stripe the same you can do with bank accounts/wallets.