The delivery analogy doesn't hold. It is not illegal to sell the same product or service at different prices, as long as the distinction is not driven by payment method.
There is a distinction in service level for the delivery analogy.
It is illegal to sell the same product and service at a different price from what is advertised to a customer under advertising law.
That is what sank the computer shop I worked in.
Customer was told on the phone by the workshop manager that something was £18
When he arrives the shelf edge stated £20 as did the POS terminal.
The customer challenged this through Trading Standards and the Sheriff agreed that as the product had been advertised to him at £18, therefore £18 is what he should have been charged.
The mistake the workshop manager made was that he read out the price from the internet sales price list; which was legitimately different because it's a different service provided for the same product.
However there is nothing to stop you being advertised a different price from me simply because you are you and I am I.
Advertising two prices to a customer at the same time for the exact same service is going to be problematic; and with payment surcharges banned the only way available to differentiate other terms of the service provided.
Providing the free envelope and stamp for return of brevet over requiring an SAE to be provided is a distinction in the service level provided.
Licensing law goes further, requiring all prices charged under on license to be consistent and published at all points of sale; though that is devolved so may differ in England.