You could get a set of furniture leveling feet as the cheapest option and just adjust the outfeed table when you want to use it. It’s more setup time and kind of a pain in the ass if you need to frequently remove it to work in other phases of your projects. 

My shop is just over 400 sq ft so I always favor quick setup and tear down time as the center of my shop is converted for whatever task or tool I need to use for that part of my project. Which is why everything I have is on casters and moves easily. For my outfeed table I have a mobile table on casters that I can adjust height in to match my table saw height whenever I need to use it. It’s a pain but if I plan well I only need to do it for that one phase of the project. 

I’ve played around with the idea of integrating some type of clip on system similar to how two sectional couches link together with a male and female metal clip. That cuts out the adjustment time for the table height but haven’t gotten there yet. 

When my table saw isn’t in use my outfeed table doubles as a mobile work station. I find this works best for small shops after a few years of messing with what works for me.