Counterbore a bare hole

Because of some mechanical issues we believe that we need to counterbore 2 holes on one of our 2.4mm pcbs. We want to counterbore to a 0.5mm depth. The unplated hole is 4mm diameter and the counterbore diameter is 7.5mm. We have never done this before and want to know whether this is an accepted practice and how much this would impact cost ? What should we be careful of?

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