Is it possible to perform donation with In-App purchase for amount the user want to donate? With last update Apple rejected my app, because I have donation process with PayPal through Safari. They said to implement in app mechanism. But is it possible for users to donate with In-App with amount the user want? I would like to avoid creating Purchase Items for every amount I want to use.
No, in-app purchases can only have predefined prices set in iTunes Connect. You can charge the same in-app purchase multiple times, meaning you can calculate the donation to the nearest Tier 1 multiple and repeatedly charge it, but you can’t have custom tiers. Using Apple Pay to get around this is prohibited by App Store guidelines.

Here’s how PCalc shows the various donation levels: