I use to sell software exclusively over the phone, deals in the 3000-100,000 USD range in the early 1990’s Silicon Valley. We would also use the coffee mugs or T-shirts to not only avoid discounts but to get the PO faxed before the end of the month/quarter. It truly did work more times than not.
Once I went to the fax and the CEO had my 9K PO and shaking his head. I asked if something was wrong and he replied that he would never send that much money in such a manner. I replied but what if I gave you a coffee mug?
Bemused he handed over the fax still shaking his head-NO.