I'm a S&T user of 8+ years...in conjunction with a stand alone GPS for our travels.
I use S&T to PLAN the trip with the route the way I want it...I even print out our 3+ week summer trips in the "strip map" format with each day on a new page...and we take that with us.
What I do is similar to those above in that I simply program my GPS and alter the route to MATCH my S&T route by way of waypoints/etc...this way I have my printed out maps for my wife to use and I have the GPS that should be programmed to match that print out.
I spend 5 minutes the night before we are hitting the road to program the next day's travel route in the GPS...
I gave up trying to export and import the .gpx files...and simply spend the few minutes each night to get my routing set in the GPS...I find it much more reliable and it keeps me in control...LOL
I'm still a huge S&T fan this way I can scout out the best routes and plan on the big screen of the home computer...then once we are on the road...I can program the GPS accordingly each travel day...we've been doing it this way for many years and it seems to give me the most flexibility with the least variability and allows me to set MY route the way I WANT TO versus Garmin software that will put you its way to save 0.001 miles or 0.000001 seconds...ha-ha...
Good luck!
Joe.