Rule R78 in Section 4 states you can use a pneumatic cylinder. A linear actuator is just a guided pneumatic cylinder, so I think you're okay.
However it might be difficult to find one with enough force for under $400. Remember, we can only use 60 psi, so if you needed 150 lbs force (robot, battery, bumpers & a safety margin) you'd the need piston area to be 2.5 square inches (150/60). That's 1.78" diameter or bore. The next common size is a 2" bore, but those are very expensive, WAY over the $400 limit.
Here are a few listed on Mcmaster Carr.