Can I use HubSpot's custom objects with Paycove instead of line-items?

Paycove does not currently support custom objects. Custom objects are meant to be very flexible, which makes them difficult to work in Paycove. Each custom object's property name would be different, so Paycove would have to know which property is the value, quantity, etc.
We do support custom fields which you can use to customize your deals.
Check out this video to learn more about how custom fields work.