Multiple Carriers/Suppliers Back-to-back model

[DCS4U]

Hi, so I'm trying to get my head around how is best to approach and configure my shops carriers and shipping.   First of, I have 3 suppliers (at the moment), 1 is based in Switzerland and will use UPS, the other 2 are based in the UK (my home country) and will use our local/national carriers DPD & TNT.  1 of the UK suppliers already has a fully functioning online store with checkout/shipping, the other only has a request to quote option on all their product.

 

So my question is, how am I best to go about configuring my shipping?

Would this be managed by a module or would I use carrier API's? Could those API's be where my one of my suppliers online store points to? I do I have to have my own account?

I have recently come across a module called Easy Post Pro, but I'm unsure if this can manage my shipping like I want it to?

I'm just a back to back order site, but I need to know my site is calculating shipping correctly based on my suppliers shipping.  I don't need my site to actual trigger shipping as my suppliers will handle that through their accounts and just invoice me.

I'm really trying to get my head around this will work so if anyone has any advise, I'd greatly appreciate it, thank.

[DCS4U]

If anyone has any advise or previous experience/understanding, I would be very grateful ?