When declining an invitation or expressing feelings of disappointment about missing an event, there are a number of synonyms for "sending regrets." Some examples include "apologizing for one's absence," "regretfully declining," "expressing disappointment," or "respectfully declining." Other options include "declining with regret," "politely declining," "sorrowfully declining," or "sending apologies." Ultimately, the most important thing is to communicate one's regrets or disappointment clearly and respectfully, while expressing appreciation for the invitation or event. By doing so, one can maintain positive relationships with others and show that they value their time and efforts.