You dont need mapcare. If your infotainment system has that sd card 6PO919866 in either of the card slots, that's where your sat-nav gets all its info from. That company I mentioned supplies cards that (if activation was ever required) simply slots into your sd card port. Give it a wee bit of time to update anything in the head unit's firmware, then you are good to go!
Think of it like this - your dealership is preying on your ignorance of how this works, so they exploit you by telling you nonsense, such as:
you'll need to leave your car with them, as it's "very technical".
Its expensive because your head unit is actually a small computer, and needs reprogrammed, etc...
Worst case - the card doesn't work, so you send it back for a refund. What's to lose?