First off, any sort of payment would not be a donation and you must be careful not to name it that unless you are a registered charity. There are laws regarding what can and can’t be classed as a ‘donation’ as it’s an important distinction, particularly in tax law.
As for what is/isn’t allowed, there’s a lot of restrictions around sending users off-site for buying products/services, such as 4.6.3 Off-site links may not refer users to sites where the main purpose is to buy products or services, unless the link is to Twitch or Amazon properties (except as allowed in 4.9).
You can’t charge bits for upgrading a service, as a streamer can’t use bits in an extension running on their own channel, and even if you are an affiliate/partner and have Cheers on your channel, it would be against Twitch’s Bits Usage Policy for cheers to be used in exchange for services.
Even if Twitch allow you to link off-site for payment processing, they may have issue with you using PayPal, as apposed to an Amazon system such as Amazon Pay.
One example of an extension that successfully uses a freemium model is Crowd Control, which provides additional features for broadcasters/viewers who subscribe to the developers Twitch channel.