Collective Purchase System
I often organise collective purchases of HVAC filters in my neighbourhood. It started as a WhatsApp group where people signed up and paid when they collected the goods. As more people joined (we're now well over 80 households per round), so did the risk and admin of fronting the money. So I built a small system on top of the API of my bank, bunq. I set up a round with items and prices, share a link or embed the signup form in an existing site, and everyone who signs up immediately gets a payment request in their banking app. I can see who paid and who didn't, and refund someone with one click if they drop out. It turned out to work for more than purchases: I'm using it for our neighbourhood day this Saturday for dinner signups and collecting information like dietary preferences.