How to model data objects being packed in a message from one participant and then unpacked by another participant

[Torsten Knodt]

Hello,
I try to convert one of my companies processes from very informal flow charts into at least semi-formal or more formal BPMN diagrams.
I have read the standard and examples and found already a lot of help there, but now struggle at one point.

In a tendering interaction between customer and supplier, I want to model that the customers, for us hidden, "Request for Proposals" process sends an "Invitation to Tender" or "Call for Proposals(s)" to us (Message), which contains several work products (Data Objects), developed in this customer process.
Then our "Supplier Tendering" process receives this message, stores it and then extracts the work products from there for further processing. 
I am not sure how to model this packing/ unpacking in a way that it is clear, that this are the work products provided by the customer.

Could someone perhaps give me a hint or tell me where I can find a good example?

Thanks in Advance,
     Torsten Knodt