While slow as fuck, i have no had any issues get any of my rebates from mass saves for my HVAC installation, washing machine rebate, and heat pump installation.

A big part of the current rebate is integrated controls, which defeats the whole purpose of mini-splits efficiency allowing for controls of individual rooms but that's off topic, so if you're unable to prove you had it installed you're not eligible for the rebate, but your contractor should be able to itemize the final invoice for you.