Even if, I close the 5G Home internet service, you charged a certain small bill

T-Mobile utilizes a “billed current” system.

The term “post paid is somewhat nebulous in terms of the actual service; the detailed bill shows the service period covered, and the bill is “issued” at the day  before the start of the service period and is not due until later in that period.

For example, my last bill was issued on July 20 and is for the current period (July 21 - August 20); it was due today (July 13  but paid on July 11 using auto-pay) - so technically I paid for past service and future service. Any added charges excluding service are actually post paid. Had I waited until July 21 or later, to cancel I’d still be liable and charged for that additional day on the next bill (prorated charge).

Regardless, the detail billed for post paid accounts details the period covered.

FWIW, this is simply a peer to peer (customer to customer) community not direct T-Mobile support. You should contact T-Mobile directly using the appropriate method:

https://www.t-mobile.com/contact-us