Just want to buy a 2nd plugin, but cant??

I just want to buy another plug in at this time. Specifically the "Appointments" plugin. I do not have enough money to upgrade and pay $500+ at this time. Although if make it in the next 30 days, am planning to do so. All I want right now, is to buy another plugin, but the system is no allowing me to do so. Its forcing me upgrade with no other option. PLEASE HELP?!?!?!