Which discounts apply at checkout?

They apply themselves. No codes. A multi-pet discount shows on the "How many pets" tile, a returning-client discount from your second completed booking with that business, and a referral reward from inviting a friend.

The largest one wins. When more than one applies, you get the biggest single discount. They never add together.

Locked in when you book. A business changing their percentages later doesn't change what you paid.