Miss Manners: The invite to a relative’s birthday dinner included a ‘suggested donation’

DEAR MISS MANNERS: We received an online invitation to my relative’s 70th birthday: a surprise dinner and music at a nice restaurant. Included in the invite is a “suggested donation” amount of $50, with the donation going towards a vacation fund.
If that wasn’t odd enough (these relatives are well-off), they also suggested that if we “feel generous,” we can add an 18% gratuity. A gratuity for what?
This feels a little like a shakedown.
GENTLE READER: Only a little?
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(Please send your questions to Miss Manners at her website, www.missmanners.com; to her email, [email protected]; or through postal mail to Miss Manners, Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO 64106.)