Over at aircondition.com they call those $29.99 packages "Death Kits." While thay may be a bit of hyperbole, it is definately better to flush out the mineral oil and vacuum the system. Excess oil lowers the performance of the system, air in the system stresses the compressor, and as Larry has pointed out, water is a long term death knell. The reason the shops are more expensive is because they are doing it right - they don't want you back in a year complaining.