I recently traveled to Italy and purchased "City Navigator NT"MicroSD/SD Card for Italy/Greece for $69.99 for my Garmin Nuvi 255. When I went caching I just popped the micro card into my GPSmap 62st. Worked like a charm!
I checked the Garmin website and it says that the software is compatible with your Nuvi 550. Here is the link... https://buy.garmin.com/shop/shop.do?cID=253&pID=8424
Laurie ~ bugeye59