The pay online functionality is great. However this should be configurable on a per invoice and per customer basis, not just a system-wide setting.
This enables us to ask the customer in advance how they would like to pay and make an invoice up based on that (so we can add card fees etc if necessary). Customers do not always want to pay the same way, so it makes sense to have this as an option.
Also, it should offer multiple payment methods not just one - some people LOVE paypal, others HATE it. So just offering paypal, or just offering stripe etc is not a good way to do it.
As an overall comment, I think any feature you implement on KashFlow should have redunancy so it can be overrided at any point. So having a system wide setting is fine, but it makes sense to be able to override these settings on a per customer/supplier or per invoice/purchase basis.