Absolutely. InmateAid is not a bundled subscription and there are no required purchases beyond what you actually want to send.
If all you need right now is a postcard, that is all you buy. No phone service, no letter package, no minimum order. The platform is built around the idea that you use what you need and skip what you do not. Some people use only the phone discount service. Others send only letters and photos. Others use everything available. It is entirely up to you.
A single postcard is a perfectly complete order and gets processed and sent the same way any larger order would, with InmateAid's Florida address on the return and your personal address kept off the envelope entirely.
If your needs change down the road and you want to add phone service, letters, or photos, those options are always there. But there is no pressure and no package you are required to purchase first.
Head to InmateAid.com, set up your inmate's profile if it is not already there, and send the postcard. It is a straightforward process and your inmate should have it in hand within about a week.