Hi @Steveknz 
If you don’t find anything ready made, since shopify-flow app has an add-order-line-item action it’s feasible to do something like this in tandem with customizations for the theme or order-status page and using the send-http-request to get an entropy source(randomization) from a third party api after an order is created. Just remember to develop and test rigoursly to prevent the system from being abusable.
https://help.shopify.com/en/manual/shopify-flow/reference/actions/add-order-line-item
This also means if a post purchase “spin” app can’t specifically add an item then shopify-flow could possibly be utilized for that part of the process such as through monitoring order tags, or metafields, etc.
Good Luck.