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If they where repairing the dryer this is part of the sealed system that should have been covered under Your warranty.
The only fees You should have had to pay would be a service fee but most companies I have delt with dont charge on warranty because they have contracts with the manufactuer.
If You only have a partial frost pattern and it doesnt get worse or better You have one of two possible problems, either a restriction in the cappilary tube or an inefficient compressor, more likely its the compressor.