SumUp

I haven’t head of it.
Can you ask if they can use another method like Paypal?
I had a bad experience with Venmo, so my preference is not to use those electronic transfer apps.

It is probably as good as any other method, most issues usually are not with the method of transfer, but with the person receiving, basically not delivering as promised.

So if you trust the guide, I would work with them on the transfer and look into the app.

It's a card processing and payment collection service. I don't see a problem using it.

SumUp appears to be like Square. Just give them a major credit card and if you run into trouble later, you can chargeback. Never give cash.

A lot of smaller vendors (maybe bigger too?) in France use SumUp. I've only used it when paying by credit card though. As someone else mentioned, it's similar to Square.