Some post offices have used those stickers in place of registered stickers before, though I don't know how likely registered would have been used, either.

It's possible the seller has access to the stickers for their parcels and are using them for letters, too, going by the postage paid stamp, they would at least have an account, if not a negotiated contract (stickers like that are an optional, paid extra for sellers on contract rates, I believe, but as it states, they are supposed to be used strictly on parcels. A letter sized article can be sent as a parcel, I've received a few letters that were sent as parcels via eParcel, but without knowing the postage paid, it would be impossible to tell which service - letter or parcel - was actually paid for).