Need advice on my uc50 application form and earnings advice

Hi im seeking some help regarding my UC claim and am hopeing someone can advise me i recieved a text this morning informing me of a uc telephone appoint coming up in the next 7 days.I assumed this was my uc50 form telephone assement so phoned them to ask for it to be recorded.They informed me that my claim had been shut down due to the form being sent back late.The date for it to be sent back by was the 12th of this month,i sent it back on either the 7th or the 8th through my local post office and not a letter box. Is it classed as my fault for it arriving late? and is it possible the phone appointment i have coming up is to discuss this?

Thankyou for your reply,no i did not use tracking post and on the post office website they state that a saturday is included in the delivery days.They never said what day they recieved it on. I also posted the same question on my journal asking if this what the upcoming phone meeting was about and the reply i recieved was "
The appointment on the 29/2/24 is to discuss your commitments to Universal credit." so i am guessing it is better to have the phone appointment and take it from there.