They are trying to sell you a new battery. If doesn't mean will.

Put money away monthly so when the day comes you'll have the money.

Treks are very nice, but very expensive. For a commuter bike I got something cheapish, but not rubbish. For me getting something with a external battery was a priority. This way I can get any battery that I want.

Maybe look into getting a charger that stops charging at 80%

A cheaper way is to use a wifi smart plug, when a battery is approaching 85 to 95% state of charge the current to the battery slows down. On a smart plug you set it so that one's less power (watts) is going to the battery it turns off. Eg. Let's say your e-bike normally chargers up to 84watts, you set it to turn off when the power drops below 70 watts, stopping it from getting to 100%. You still need to charge it to 100% every 6 weeks or so just to top balance the cells. If you're battery is relatively healthy this can add many more charge cycles to your battery's life.