T-Mobile has been encouraging employees to use customers' T-Life app for transactions for quite some time. The company reportedly offered a bonus for adherence to the practice, and employees were reportedly reprimanded for not meeting targets.

Starting today, all COR and TPR stores need to be at 60% usage for T-Life for adding voice, watch, and tablet lines and device upgrades.
If you want the old retail experience, the only way we can be in compliance and use our “legacy” tools is by taking your required payment in cash.tva_raylan, Reddit user, April 2025
They say that if you choose to pay in cash, stores will be forced to use their legacy tools, freeing you from having to use T Life while at stores. This tip is only for existing customers, as new customers don't have to worry about having the app installed. While stores vary in how much they choose to do through the app, with some exempting bring-your-own-device (BYOD) transactions and watch and tablet lines, ultimately they will all likely require you to use the app for most of the things. Thus, if you don't want to be compelled to use the app, it's best to carry cash when you visit a T-Mobile location. That's because the T Life app doesn't let you use cash. Otherwise, store reps will have no choice but to ask you to use T Life.