Well first see if the promo works if it does and numbers make sense, you can apply the unconditional free shipping for a while if the behavioral economics of peoples choices are similar in a sense that they treat the promo and actual shipping change the same way, you can leave unconditional. shipping as is, if not revert back to the initial pricing.
3 Stage:
-
Promo test
-
Change shipping rate on the website, and try unconditional for a while
-
Keep or revert back
Hope this helps
in 