Priority requires one 50c priority label regardless of the postage cost.  So your $1.40 letter will now cost you $2 plus 50c f you want it to go priority.  A $2.10 letter will cost $3 or $3.50 priority and a $3.50 letter will cost $5 or $5.50 priority.  Priority letters still don't have any sort of tracking, so if you want tracking you will need to add registered post at $3.75 as well as the extra 50c for priority as registration doesn't alter the speed of the service.