Export payment terms for Angolan hardwood

Question

Hello,
We are the newly formed UK arm of an Angolan company exporting Hardwood.
Our buyers are not happy placing deposit/cash in advance with us and our owner is not happy to use LCs.
From our standpoint, we need to get paid as quickly as possible after the containers are loaded (need cashflow) but appreciate that the buyer needs security also.
I have been offered various different methods but in honesty don’t understand how they affect us.
I have offered a cash against loading i.e. we pack containers, send loading pictures, packing list and BL copies to client and then they pay us within 72 hours. The buyer said we still hold original documents so its not worth the risk for them, I simply dot understand what this means.
As you may gather, I understand a little but not a lot bout the process and am finding it very difficult to negotiate.
As mentioned, we need cashflow and to be able to get paid as soon after loading as possible but I need a method which gives the buyer security also.
I would greatly appreciate any advice and guidance you could offer.
Kindest regards,
Ash