WishKeep is made by one person, and every letter is read by that person. Questions, bugs, a shop whose page will not read, a word the categories keep getting wrong, or something you wish the app did — all of it goes to the same address.
The quickest way is from inside the app: Settings → About → “Write to the developer”. The letter opens with the version, the phone and the language already filled in, which is most of what a bug report needs and none of what is in your list.
What you did, what you expected, and what happened instead. If it is about a shop page that came back empty or wrong, the link itself is worth more than any description of it. A screenshot never hurts.
Moving to a new phone. The list goes across through the copy in iCloud — open the settings on the new phone and choose “Restore from iCloud”; the app offers it by itself when it starts up empty and a copy exists. A backup file saved from the settings does the same thing without iCloud.
An item came in without a price or without a picture. Some large shops answer a program differently than a browser, and on some sites the price is drawn by a script and simply is not in the page. Both can be typed in by hand on the card, and “Photo” puts your own picture there in two taps.
Taking a published link down. Open the list, tap the share button and choose “Withdraw the link”: the page and its pictures are deleted at once. A link nobody withdraws stops working ninety days after the last publication anyway.
A reservation on a published page disappeared. The page recognizes a guest by a cookie and by nothing else, so a browser that cleared its cookies, or a different browser, is a different guest. Nobody can release somebody else's reservation — not even the owner of the list — but the owner can remove the item itself.