65 PSI sounds low to me too. I'm not sure what static pressure should be on this R-134a system, but the rule of thumb for R-12 is that static pressure equals temperature (F), and R-134a has a generally higher pressure/temperature curve. So unless it was really cool out, 65PSI sounds low.
Is the compressor engaging or not? If not, it strikes me as unlikely that adding refrigerant will affect that problem. Almost every car - older MBs & just about everything else I've ever worked on - cutout the compressor when static pressure falls below ~30PSI. This function both protects the compressor in a low charge situation and also serves to shut down the compressor when outside temperatures fall below freezing.
If the system is running but not cooling, don't just go adding refrigerant if you don't have a set of guages. If you can't monitor both the high & low sides, you won't know what's going on nor whether you are likely to damage the system.