I wanted to test a workaround using a module I don't currently have (AR) but I don't want to buy the On Site version only to find that the workaround doesn't... work.
Could I subscribe to the online module for the test period and if it fails let it lapse or if it works buy the on site version?

Your dealer can install an evaluation copy on premise that lasts for 60 days. But you would want to make sure to test it in a copy of your data, since creating the data in your live company might cause problems if you decide to let it lapse.

oh wow, that would be perfect! I didn't know that was an option. Thanks Dan!