Add custom rates

🚧 THIS IS A TICKET MIGRATED FROM UPVOTY. The original ticket and comments will be available for a limited time over at:
https://abrp.upvoty.com/b/request-a-feature/add-custom-rates/
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There are some apps, like Bonnet, that have different rates to other subscriptions - Bonnet in particular has Geniepoint and Ionity at 35p, 30p, 29p and 25p per kw depending on the package you take out.
Having the ability to edit unit prices would mean no longer having to manually work out the journey cost