We're looking for a superb web development company to integrate Paypal with ActiveCollab's Invoicing Module.  ActiveCollab already gives us the ability to invoice clients and send pdf invoices.  We need to extend this functionality to receive paypal payments for invoices as well.

When clients receive pdf documents we would like to add a paypal (pay now) button that will allow them to go to paypal and pay for the invoice in full.  Once the invoice is paid in full, paypal calls back to ActiveCollab and marks the invoice as paid. 

The API documentation for ActiveCollab can be found in the link below.  Thank you
http://www.activecollab.com/docs/manuals/developers/api