StudioVulcan

I've been doing something for a couple of years now that may help you guys, but yes, it doesn't seem as professional as being able to EITHER: Create a manual order for a client to use the link and go ahead and pay the order, or be able to order with a custom pricing that they type (that we've agreed to).
We're really just asking for invoicing and how fucking difficult is that? lol.

Anyway, I added a product to my store that is $1. I then allow the quantity field to be edited and have them change THAT to the $ value we agreed upon. I've never seen additional fees doing this as it's a digital product.
I then in CSS convert the text "quantity" to what ever text you want. I went with "Agreed upon price". It's the only way i've been able to get around it.

Squarespace markets themselves for creatives and service sellers but come on... No manual order creation for easy buyer payments, or just straight up easy invoicing. Come on, this is 2021, we've been asking for it for years. It's not a hard thing to do when most of the bones are already in SS's system.