As a Seasonal, I don't think you can join the Union unless it is different in your area. IF you are asked to stay on permanent, I think you have to get 90 days in before you join the Union. Seasonal work does not count towards your 90 days and it's 90 WORK days... Some think come April they completed their time - wrong - more like May... There really is no reason to join the Union and pay dues UNTIL you complete your 90 days... After that, I strongly suggest you join the Union.
If you do a good job you MIGHT be asked to stay on permanent BUT... It might not be as a full time driver, could be wherever they have an opening. You COULD be placed in the unload (equal to being a private) throwing boxes out of a trailer part time for $9.50 a hour. From that point it could take five to ten years before you move up to a Driver position. It all depends on SENIORITY and how many retire, resign, move to a new position or get fired...
Come in about 30 minutes before your report time and get to know your preloader, they can tell you about your load - any Haz Mats, bulk stops, Irregs, Blown out self, etc. Don't interfere with their working or expect them to be able to stop and chat. Your preloader is key to you having a good day - You don't want to be searching the 2000 shelf for a package to find out it is loaded on the floor. December 24th, give your preloader a $10 Subway Gift Card just to say thanks - You don't have to but it's nice...
Get to know the drivers in the package cars next to you that your preloader loads... When you are loading three package cars - 5E can look like 5B... There is ALWAYS a chance you will have a misload that belongs to them. If you have their cell phone numbers you could meet them and hand off a package or just meet for lunch.
Get to know your Union Steward... Go to a couple General Meetings at the Union Hall, there should be one in November and December. The General Meeting are open to friends and family and most have a nice Christmas party with door prizes. Just because you are a seasonal non dues paying member doesn't mean you are unwelcome. They want you to be successful and join... You are protected by the Union and enjoy all the privileges under the UPS TEAMSTERS MASTER AGREEMENT... The Union Steward is also key to your success....