The 3 big wholesalers (Amerisource, McKesson, Cardinal) also own/operate PSAOs that provide various value add services to independent pharmacies.
I would say PBMs are far more creative with arbitraging and financial engineering, because they deal directly with plan payers, pharmacists, distributors, pharma, DME mfr, and patients.
They offer price guarantee/inflation protection, rebates to payers; they have very granular reimbursement agreement with pharmacies along with performance guarantees or penalties depending on how much generics they dispense, there is copay assistance program with pharma and patients.
PBM is a complex business, but there's no huge pool of money sloshing around. Money is in pharma and overall healthcare systems.
This is a long game for Amazon.