Add the product unit type descriptor.

[dd32]

As prompted by Stripe, we need to add the unit_label to the checkout process to ensure that Stripe receipts properly show what is being purchased, to ensure that we continue to receive the most favourable Visa/Mastercard rates.

This field does not appear to be properly documented yet, and I'm not sure what values should be present, so this needs testing.

Reading:
https://support.stripe.com/questions/understand-the-visa-commercial-enhanced-data-program-%28cedp%29?email_id=em_km8f3fl4cbstlcesjsikf63zcnuy2i

This also suggest that we should be sending some additional data such as a product_code and line_item[tax] which it doesn't appear that we're doing.

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