This can be a complicated answer depending on how the practice is set up, especially if each location has a separate TIN. In addition as mentioned above the accrediting organization absolutely needs to be contacted to ensure compliance. The date of collection is always the date of service in lab testing, not the date that the sample is analyzed. Now there are some exceptions for molecular pathology, but for drug testing, it is always the date the patient is seen. So in this situation, if all of the check boxes are checked- TIN, and accrediting then you would bill POS 11 and one unit of 80307.
I would caution that there are multiple potential issues with the model that you are currently working under. First, a presumptive drug screening is meant to be used in the treatment of the patient (meaning while the patient is there in the office and the provider uses the results that day (I know, I know, it takes 52 minutes for the analyzer to run, don't kill the messenger payers are on this) then as a physician office lab, you would then base any definitive testing on the results of the presumptive. My experience is that the laboratory consultants who set these labs up don't tend to explain to the physician the issues that this can cause. Second, make sure that you do temperature transportation studies on your collection cups, or this could be a major CLIA violation, and ensure that the samples being transported are secure from a HIPAA perspective, you can't imagine how many cases I have dealt with where the MA in charge of transporting the samples left the samples in their car overnight, or the specimens didn't have two identifiers that matched the cup to the paperwork.
Hope this helps, I would highly recommend your practice get in touch with a healthcare attorney that has clinical laboratory experience before continuing to bill for these services.