Hi Tobias,
Assuming that we are talking about Special Customs Procedures (IPR, OPR), you will need to set up an ECPID. Then assign the ECPID in the Product Master record. There you should be able to see the flag for using the External BoM.
Yes the BoM is exploded during document processing - internal or external.
Billing Documents are fine for exports, as you say.
Bop ID is necessary in the BAdI to hold everything together.
Hope that helps,
Dave.