The book is already written, except for the secret ending, so people are wondering how they can become a character.

Simple.
As a patron, you become mentioned at the front of the book as a Patron and a member of the nobility of Harrhein. Ranked according to payment, from $25 to $75 for a Dukedom.
For $75, your name is simply given to an otherwise nameless character with little role but your name is suitable for their position.
For $200, you can choose a Western sailor, either sex; a male viking; a South Sea Islander; a Chinese or an Indian. The latter appear in the closing chapters. You may describe the looks and basic character and this character will take the place of one of these existing characters.
For $500 and $1000, as above but an additional short story of 5000 or 10000 words is written with the character as the hero, and the character will have a more central role in the story. They are more likely to be officers or people of rank. This does require quite a bit or re-writing, which is happening constantly anyway.
$2000 allows your name to be substitued for one of the major characters, Princess Asmara Starr, disguised as Sara; Suzanne Delarosa; Padraigh Connorson or even Sung Bai Ju - though that is her title and your name would become her real name. Clearly, it needs to be Chinese! The further advantage is that these central characters may be unlikely to die and thus feature in the next book...
Only two days to go! Sponsorship closes on Tuesday 17th. We have successfully raised sufficient funds to start the marketing ball rolling, anything more will speed it up!
https://publishizer.com/in-search-of-spice
Best wishes and thanks for your help

Rex