Squarespace is dangerously close to being a perfect lite weight CRM with digital invoicing. There are 2 simple features that would make it perfect that are missing.
1. When generating an invoice, it should be possible to populate line items from existing product/service inventories and have the relevant inventory update accordingly
2. When viewing a client/contact "profile" under the "contacts" tab, there should be a button to generate an invoice.
If these things were implemented it would make the invoicing feature as good as anything in the industry