I can understand not wanting to pay twice.
Are you sure you're remembering this all correctly, however? Like I said I don;t remember 10.6 ever being a digital download under any circumstances. The updates (10.6.0 to 10.6.1 to 10.6x) were digital but going from 10.5.x to 10.6 always required a DVD. Maybe they sent you one to go from 10.5 to 10.6 and then you updated?
If you take you model identifier (found in System Information) you can use that to see exactly what OS version was shipped with the system, might jog your memory.
Go to iMac10,1 - Lookup Mac Specs By Serial Number, Order, Model ... to get all the original specs for the system. Apple will send you the DVD for the original OS for a shipping charge. If 10.6 came out that soon after that system might be listed as having 10.6.