This is the shortest path to one mapped OS Services Booking hold that lands the correct J2Commerce product in the cart with Continue to Checkout visible. If OSB or J2Commerce is not installed yet, stop here and finish those first; Bridge will not install them for you.
Confirm OS Services Booking can select a service and show timeslots without the bridge. Confirm J2Commerce can add a test product to the cart manually. Confirm Joomla 4, 5, or 6 with PHP 8.1 or newer. Download plg_system_rca_osbj2c from https://recreatinganew.com.
Open System, Install Extensions, and upload the Bridge ZIP. Enable System - RCA OSB J2C Bridge if Joomla left it disabled. Open System, Plugins, open the bridge plugin, and locate sid_map on the Basic tab.
Enter one pair per line as sid=product_id, for example 2=1, where sid is the numeric OS Services Booking service ID and product_id is the numeric J2Commerce product ID. JSON object form is also accepted if you prefer that syntax. Map every service you intend to book through checkout, but start with one service, prove the path, then expand. Save the plugin.
Clear Joomla cache under System, Clear Cache. Use the normal OS Services Booking UI or embed osbtable in an article using the shortcode form your OSB documentation supports. As a frontend user, hold a valid future timeslot on the mapped service. Confirm the mapped product appears in the J2Commerce cart and Continue to Checkout works. Do not complete a live charge on production until you verified on a safe test order.
A mapped hold puts the correct product in the cart and checkout is reachable. An unmapped service should error on the bridge path; that is expected behavior, not a mystery bug. If the wrong product appears, fix sid_map IDs before you blame OS Services Booking.