I was praying Asr and I had already done Salam and I was at the part where you pray and do dua. I burped and said “excuse me” is my prayer invalid? Read
You can hardly invalidate a prayer which has ended.
And the prayer ends with the salam or tasleem:
The Prophet (ﷺ) said: The key of prayer is purification; takbir (saying "Allah is most great") makes (all acts which break prayer) unlawful and taslim (uttering the salutation) makes (all such acts) lawful.
(See for example in Sunan abi Dawod 1 & 2, Sunan ibn Majah 1 & 2, Musnad Ahmad and Jami' at-Tirmidhi 1 & 2)
Tahleel تحليل or making it lawful means, accomplishing it (and earning the rewards). So anything you've done after this has no impact on the prayer. But one should consider repenting.