A merchant frequently runs several discounts at once, each valid only through a particular route. The restriction exists because the cost of a customer varies by channel.

Acquisition cost differs by route

A customer arriving through a paid placement, a partner site or a comparison service brings a commission or fee attached to the sale.

A customer arriving directly brings none. The same discount therefore costs the merchant materially different amounts depending on how the visit originated.

Setting a single discount across all routes means either overpaying on the expensive ones or underpaying on the cheap ones, so the codes are separated instead.

Codes can be intercepted late in the journey

A shopper who has already decided to buy may pause at checkout to search for a code, and the site that supplies it can be credited with the sale.

In that case the merchant pays both a discount and a commission for a customer it had already won, which is a pure loss on an otherwise complete transaction.

Channel restrictions are the direct response, ensuring codes distributed through such routes are the ones the merchant is willing to pay a commission on.

Attribution rules decide who is paid

Most affiliate arrangements credit the last referrer before purchase, which advantages whoever appears closest to the checkout regardless of who created the demand.

Merchants adjust for this by excluding certain partner types from commission on code-led sales, or by reducing the rate where a code was applied.

The commercial terms and the code restrictions therefore have to be designed together, because either one alone can be worked around.

Partner-specific codes make policing possible

Issuing a distinct code to each partner lets the merchant see exactly where redemptions came from and whether a code appeared somewhere it should not have.

Codes that leak beyond their intended channel can be retired individually without affecting the rest of the campaign.

This is why the same offer often exists under several different code strings, each functionally identical but traceable to a different source.

What shoppers experience as inconsistency

The visible result is that a code found in one place fails elsewhere, or that different sources advertise different discounts for the same merchant simultaneously.

This is not error or expiry in most cases. It is the merchant operating several parallel offers with deliberately incompatible conditions.

It also explains why the route taken to a checkout can change the available price, even though nothing about the goods or the basket has changed.