DearXXX:
Itismygreathornortogetyourinvitationtothe10thanniversarypartyofyourcompany.Iamsureyouwillhaveagreatmomentwithyourcolleaguesandyourclients.Therefore,Ishouldsay,"Congradulations!"Iamsoproudofthedevelopmentofyourcompanyandhappywithyouallguyswhousedtogivemegreathelpinthebusiness.Butit'sapitythatIwillhaveabusinesstriptotheothercity.IamafraidthatIcan'tattendtheparty.Iamsosorryforthat.ButIwillsendmypresenttherebefore-handtoshowmythanksandcongradulations.
BestWishes!
SincerelyYours,XXX