Whether the app isn't currently meeting your needs, or you recently bought it by mistake, please request a refund from Apple, putting in a note that GoodNotes approves the refund. Please note that all refund requests are handled by Apple only and not by developers, which means there's little we can do in the unfortunate case of a rejection [1]:
All Transactions are final. Content prices may change at any time. If technical problems prevent or unreasonably delay delivery of Content, your exclusive and sole remedy is either replacement of the Content or refund of the price paid, as determined by Apple. From time to time, Apple may refuse a refund request if we find evidence of fraud, refund abuse, or other manipulative behavior that entitles Apple to a corresponding counterclaim. Terms related to Store Credit and gift cards/codes are available here: https://www.apple.com/legal/internet-services/itunes/giftcards/.
For best results, we recommend contacting Apple support by phone to explain your situation in more detail.
If you're granted a refund, please remember to export any necessary notebooks in your GoodNotes library before deleting the app.
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Reference:
[1] https://www.apple.com/legal/internet-services/itunes/us/terms.html