Dear Romain,
As you mentioned in your post that in access seq you are also considering document type . you are using pseudo order type to create delivery & billing with reference to delivery . please check in billing document type in account assignment/Pricing which document pricing procedure you are using , is it the same one which you assigned to order document pricing procedure.
I think, due to consideration of document type you need to maintain entry in pricing determination transaction against billing document type. also in Copy control you need maintained D
Hope this will resolve your issue
Regards,
Milind