You CAN print labels at home for first class parcels (not parcel post). Any printer, no other devices or software necessary. The interface is tricky but keep looking and trying, it can be done if you tick the right boxes and don't leave any forms blank. All options will eventually show up.
I would suggest writing a computer file containing a note to yourself reminding of the procedure for the next time. I had to do that for 2 or 3 things on the USPS site (custom forms etc.). I don't have notes for finding first class because that is not a problem as Seth posted.
You *can't send foreign parcels at the cheapest rate (first class) online.
You can send foreign parcels for the higher option rates including doing all customs forums.
You can send domestic parcels first class online and print labels and postage (its tricky).
You *can't send parcel post parcels (neither pirnt labels and print postage) online.
You can schedule the mailman to pick up priority packages from your door online, but only if they are priority or express (not first class parcels UNLESS you also have at least one priority or express parcel to also pick up at the same time (in that case they will take them from your doorstep all including the first class). The mailman will leave a receipt saying he picked up parcels and how many but no other information will be on the receipt.
It is possible to avoid lines at the post office even though all the blue pick up boxes now have small openings that won't take parcels. Just schedule a carrier pick up (for the next day, you can't schedule for the same day).