PMS & Own Hostel Website Booking System - With Once-Off Pricing Model

Hi

I was wondering if anyone knows an online booking system (which is placed on your own hostel website..not a third party channel) which can interface
directly with a PMS system? By interfacing I mean that when a guest makes an online booking, it automatically takes that booking and registers it into the PMS system and adjusts inventory accordingly. Also pricing can be automatically updated by the PMS to the online booking system (which may also possibly incorporate a revenue management system to change prices according to inventory level)

The issue is that I know such combination systems exist, but I have yet to find one with a pricing model where I can buy both the PMS and the online booking system upfront without any further obligatory commissions, transactions or support costs.

I think Micros has their Opera PMS System and MyLink online booking system which does such an interface but from what I understand they ask for annual fees and commissions, which I don't like or want.

Does anyone know of a system combination which is free of ongoing fees, commissions and support costs?

Thanks

Jon