I'm curious if you ever resolved this. I am having a similar problem with the finder printer.
The issue is the pre-extrusion although surely -70 is an ok value as it fits in the print area although Flashprint doesn't like it and rejects it. I think you're fine if you print via a memory stick (although today I tried this and it crashed my finder). I'm sure it's a simple tweak of the code although I tried changing the -70 and 70's to 69 -69 but still it rejected it. Deleting the pre extrusion code is a workaround but not a great one.
I normally use flash print but there are times (like today) simplify does a slightly better job for some reason

Plus I feel that the software finally pays its way a bit.
