Gosh people got confused on this thread, didn't they. There are a few issues becoming conflated, and the actual answer (given very early on by David Griffiths) was unfortunately ignored.
Issue #1:
The question on (nearly) everyone's lips at the moment is whether it is right that everyone else has to charge VAT on delivery packages, whilst the Post Office can continue to charge no VAT on delivering letters. Whatever your views on this subject, I would suggest that this entire debate is largely irrelevant to this thread.
Issue #2:
Multiple supplies or a single supply? A few years ago, this was the question on (nearly) everyone's lips. This has an impact on anyone in the delivery business or who delivers their product.
If you sell goods and deliver them as part of your contract with the customer, then you are probably making one supply of "delivered goods". If so, the price of the overall supply is subject to VAT at the rate determined by the goods.
If you sell goods and then make a separate arrangement with you customer to deliver these goods, then you might be making one supply of goods (subject to VAT at the rate determined by the goods) and one supply of delivery services (subject to VAT at the standard rate, if it all takes place in the UK).
This issue remains less than clear and, even where there is setled caselaw, it can still be very difficult to identify and follow the appropriate rules.
But this also seems not to be the central aspect of this thread.
Issue #3:
Who is the true customer of the Post Office's services? The purchase is of the mailing house service, which involves putting letters in envelopes, addressing them, stamping them up and putting them in the post.
Let's take a clearer example. If your business buys the stamps, the envelopes and provides everything to the mailing house, there would be no issue. The stamps would be purchased from the Post Office by the business with no VAT paid. The services would be bought (separately) from the mailing house and VAT would be paid.
Let's a clear counter-example. If your business supplies nothing but the contents to be posted, the mailing house will provide everything else. The cost of the entire mailing house service will be subject to VAT. There will be no separate suply of stamps or envelopes by the mailing house to the business.
Now let's complicate things. Assume the first example occurs, but (as a practical short-cut) the business asks the mailing house to purchase stamps on behalf of the business, for which the business will reimburse the mailing house. This changes nothing in the first example, provided the mailing house satisifes HMRC's list of conditions (e.g. makes a separate charge for the stamps, labels recharge as "disbursement", makes no profit/loss on the recharge).
Now let's turn to the actual scenario. It is clear that the answer will depend on how the mailing house engages with its customers. If it is buying the stamps as agent for its client businesses, the mailing house will simply recharge to each customer the amount it paid on stamps and no VAT will be added. This is because the true customer of the stamps is the client business, not the mailing house.
However, if the mailing house falls within the second example above, promising to do everything and charging one price for it all, then the mailing house absorbs the cost of the stamps as part of its own business activity. The mailing house is the true customer of the stamps. Then VAT is applied to the overall, single price charged for the mailing services.
Conclusion:
I cannot understand why any sensible mailing house would not act as an agent for all its customers in buying the stamps from the Post Office. As one contributor noted, perhaps it is because they (and their accountants) and uncertain on this issue.
It therefore seems sensible to ring round and identify the mailing houses who do not charge VAT on the stamps, and all other things being equal (e.g. provided price and quality of the main service are the same as their cometitors) use them, as they will be cheaper overall.
I realise this is a long one, but hopefully it is relatively clear?