Early in the discussion phase of Upwell Structure design there was a promise to consider NPC delivery services for Citadels. The basic idea was to spawn an NPC convoy that woudl haul your stuff on demand to and form Citadels for a moderate fee. The concept luckily did not make it into current game, so we still have guys from Red Frog saving the day for everybody else.
With the recent rebalance of Broker Fee for NPC-owned stations CCP hoped to motivate people do bring up their own Citadels and Market hubs. However, sole monetary penalty for using NPC station seems not to be enough alone to solve the issue for consumer products - ship, equipment, charges. The reason, as it seems to me lies in two tools, that simplify everyday life of capsuleers - multibuy and autofit. The latter effectively forces people to buy fits from a single station, that should contain a variety of modules in stock. This requirement limits use of this tool outside of NPC trade hubs and discourages customers from seeking that 1-2% discount that can be provided in citadel.
It is possible at the moment to use the bonus from citadel for buying things using range 1 orders set from a nearby system. Though, this is a quirky solution. No solution exists for sell orders.
I suggest addition of NPC-delivery mechanics to player-owned citadels with active market hub against a % fee per jump (i.e. 0.5% per jump). The implementation should allow owner of the station to select a target NPC station, that would act as masquerade for all sell (and possibly buy) orders issued in the citadel. The intended result is for end-customer to be unable to distinguish between orders placed on a nearby citadel from the orders in the target NPC station allowing them to use multi buy tool as is. The physical relocation of goods between stations should be handled instantaneously or with a small, not extremely annoying delay, so that end-customer is kept unaware of origin of those wares. The fee is to be paid to SCC by seller or owner of the station, who can in turn raise its own broker fee appropriately.
Spreading trading and production activities around and solving the single trade hub issue without ruining convenience tools for consumers.