Multi location shipping rate control

[DizLun]

Hi Guys,

Unsure if this has come up before, but I'm struggling to find any prior solutions - I'll try and keep it brief:

We already ship products from one location and have now also integrated with a Print On Demand shirt company. So for example: 

- If someone orders a product from us we charge £3.99 to ship

- If someone orders a shirt via our POD supplier they get the same £3.99 charge

- However, If someone orders both a shirt & a product in one purchase, I'd like to absorb one of the shipping costs from one of the locations.

But I'd still need to charge shipping if they purchase individually? Does anyone know if this can this be achieved in any way? 

Huge thanks in advance,

Ben