Hi PhilthyCollins,
The idea you mentioned could work in theory but, at present, I believe that process would be blocked by a current issue which is causing the eBay Gallery images imported by that process to be smaller than the original images such that may not be 500 px on the longest side.
Although this never proved to be particularly problematic in the past, eBay started rejecting listings which have Gallery images that are not at least 500 px on the longest side specified within the last month, so this may now cause items imported in this manner to fail unless the eBay Gallery images are updated.
However, the “Import Listings” tool should pick up whether your Promotional Shipping Rule was selected for your listings so using that approach may prevent you from having to select that option in each individual listing.
For the time being, I simply do not know of a solution that would allow you to upload your items into Auctiva in bulk that would not require making revisions on an individual basis to update your selections for at least one of these options.
-Mike