Define a 'Shoebox API'

[andrewhughes3000]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
No

Describe the solution you'd like
A clear and concise description of what you want to happen.
Receipt issuers should be able to programmatically send receipts to a location of the person's choosing. A standard API is needed for this. The person's Shoebox API should be easily discoverable. The person's Shoebox API should be configurable in a way that allows the person to use a receipt data storage location of their own choice.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Some alternatives could be: prompt the person for a 'save' location every time a receipt is created; send receipts to obvious person-controlled locations such as email or 'downloads'; display the receipt in-browser and tell the user to print or save manually.

Parallel exploration into Media Type (used to be MIME Types) registration with IANA should occur.

Additional context
Add any other context or screenshots about the feature request here.