The fees table is publicly available on the ATOC Travel Agents website: https://www.atoctravelagents.org/third-party-retailing/retailing-costsI don't know the details, but don't retailers incur a fee for generating a barcode? There may also be a fee for writing a ToD to LSM, but in terms of cost the key question would be whether they are comparable.
I'd assume these would be the same for TOC retailers too.
For ToD:
Flat 5p per CTR record inserted.
Ticket machine collections
Per CTR: 0.5% of ticket cost, with a minimum of 7.5p and maximum of 50p.
Ticket office collections
Per CTR: 3.66% of ticket cost, with no minimum or maximum
For barcodes:
The choice of...
(a) 0.35% per CTR for fares over £10, capped at 30p
(b) 0.41% per CTR for fares over £15, capped at 30p
There are also fees for eTVD (you'd need to write that the ticket has been cancelled), but I assume these are negotiated directly with one of the few RDG eTVD suppliers.
I can now somewhat understand why it'd be ToD only with this, as then you only eat the 5p CTR insertion fee as the rainy day guarantee only permits refunds/changes when the tickets have not been collected. On the other hand, £15 E-Ticket would be 5.25p at least, plus the eTVD costs to cancel the barcode.
So, I suppose, uncollected ToD is cheapest to cancel, the E-Tickets, then collected ToDs.