Freeze 12 and all other supposed drop in refrigerants are not really drop in refrigerants. They earned that name because they were compatable with the residual r12 mineral oil in the system so the system does not require flushing, and that is the extent of it, period. Meaning that no other internal modifications need to be done to accomodate said refrigerant, it doesn't mean that you can plop it in with r12. All supposed drop in refrigerants are required by law to have their own unique fittings so as to identify them visually to any AC technician so they will not contaminate their equipment. The funny thing about that is that most of the places pimping freeze 12 and autofrost and all those other incarnations of the "drop in " refrigerants don't even sell the fittings, most can't even tell you where to get the things and just try to buy the fittings to adapt from them to an AC manifold gauge set, good luck. The manufacturers of those refrigerants have done basically nothing to see that their products can be brought into compliance.
As far as an end user, it's just not worth the hassle and the fines involved just to try to save a few bucks up front.
You're better off just paying for someone to top off the r12, or if you want you can take the test online and then you can buy a can or two yourself and recharge it, or just change it over to r134a, the stuff isn't all that bad. I have ways of getting plenty cold duct temps out of r134a to where I don't need to look back to r12