Przyjęcie - odmowa przyjścia (prośba o sprawdzenie).

Hi Anna.
Great to hear from you.
Thanks very much for your e-mail.
It's very kind of you invite me to your party.
Unfortunately, I am not able to accept your invitation due to previous commitments.
It's my grandmother's birthday on Saturday.
I promised my grandmother that I would come to her for dinner so I'm going to visit my grandmother.
I hope you'll understand it.
Anyway, I hope you have a great party.
See you son.
A.

zobacze cię synu :-)

zdanie z 'previous commitments' brzmi bardzo oficjalnie

reszta ok

It's very kind of you invite me to your party <<< brakuje jednego słówka tutaj

Brak słowa w zdaniu z invite, imo.

tak, to fakt, brak to, brak.