A person sent an email on a phone, and exclaimed "pya" after sending it. Thank you.
ぴゃっ/ぴゃーっ/ピャーッ/ぴゃっと/ぴゃぴゃっと/etc is an onomatopoeia that describes something moving very quickly. さーっと/ささっと/さっと is similar. This page says it's an Osaka dialect word.
So the ぴゃ you saw describes how the email has been sent to you (quickly).