Player owned structures are currently seen as risky destinations for courier contracts due to the simple scam where a delivery is set to go to a locked down base. The player accepting the contract pays the collateral, picks up the delivery, flies to the destination only to find they’re unable to dock and so when the time runs out they lose the contract collateral.
Due to the above scam, only NPC bases are considered by many couriers. The solution is simple, if you accept a courier contract to an otherwise locked base, simply having the cargo on-board needs to grant landing access, irrespective of the base landing rules. If this was done the scam would instantly become obsolete and PO bases would become just as desirable for couriers as NPC bases.