So...that's a rebuilt trans, right?
If you just want to get it done and the money isn't a stumbling issue, it sounds like you'd never have to worry about it. But the yearly flush seems extreme ...unless you are doing rough-service towing with your truck. Then it probably makes sense. So, if those things are true, the $3900 deal (which sounds a little high to me personally, but I haven't shopped tranny rebuilds lately) is probably a good way to go for you. You write out the check and it's done ..with a guarantee. But they probably know that after 5 years you'll stop doing the $275 flush..and they'll make back almost a fifth of the tranny price in that time. And if they're wise, they'll find some maintenance items your truck needs. But again, little risk on your behalf and that could be worth it's weight in gold to a lot of owners.
Another option is to check with local Salvage yards and/or Marketplace. See if you can find one for $500-$800 with relatively low miles. Which you probably ought to be able to. Then have a shop install it? And do the tranny flushes every couple yrs. That's going to cost you time and money to do the footwork & acquire and work out a deal with a shop, and there will always be some 'risk' in a deal like that. Usually it works out great and is a lot cheaper way to go. Or have a buddy install for free. ...but you can't completely remove risk. And if a shop charges you $800, plus $800 (tranny) + $200 new fluid/parts, etc...then you're halfway to a rebuilt tranny with lifetime guarantee (Option A above). So it depends how you feel about risk ...maybe Option A makes a lot of sense. Buy Once, Cry once.
So..you have a couple options. Glad to hear you plan to keep your tk for life! She must be a nice rig. Congrats on that.
You might also plan on having the lifters changed too if you're a lifer. Or at least consider it. Those little rollers on the end of the lifters can't live forever. That's the biggest risk on the engine probably, and could save your bacon big time. And a new set of head and intake gaskets (while you're doing the job) can't hurt.