inviting
you are warmly invited to
i would be delighted if you would join us / me for
would you like to
making an appointment
i'd welcome the opportunity to meet with you in person
would you like to get together to discuss this in person?
when are you free this week?
it would be great if we could arrange to meet sometime
when are you available?
giving bad news
i (deeply) regret to inform you that
unfortunately we have no choice but to
unfortunately
complaining
your product regrettably did not meet my expectations
i am (deeply) disappointed in
i regret that i must call your attention to
i would be grateful if you could look into this
apologizing
please accept my sincere apologies for
the misunderstanding / the mistake
i apologize for any inconvenience this may have caused
i am very sorry about
sorry about
the mix up / misunderstanding / mistake.