to make sure, that it will be seen by Dev's, not regularely visit the open
Forum Sections. And others will most likely not be able, or willing, to assist
on such highly Custom Mods, just for the plain fun of it (anymore).
Sorry for the bad News, it's nothing personal ...
(Still, one might, just to prove me the contrary )
I am no longer active around here, to reach me, contact: firstname.lastname@example.org
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Alternately, you could just add a $1 product to the store, and customers can add the quantity of that item to add the shipping cost. For example, they'd add 50 of that product for a $50 shipping cost. (You could also do that with a $0.01 product if you needed exact shipping costs.)