After your elaboration of the incident, I believe your mistreatment to be both real and unjustified. Your recognition of the inappropriateness of racial slur shows that perhaps that character reduction may not happen again. That being said and putting it to rest, I believe I can offer a likely explanation for why you were treated so poorly.
I believe the reason you were treated in such a way was due to a personality shortcoming of that business owner. There are frustrating aspects for The UPS Store owners, one of which this particular owner sadly and wrongly dumped on you. Let me try to explain. Decades ago, there was the post office and UPS. If then, a person had a large shipment with certain needs, UPS would be a more conducive shipping company. Unfortunately, the package would have to be driven many miles 'downtown' to a hub facility. In 2001, the UPS corporation saw an opportunity to access more of the small package market by purchasing the Mail Boxes Etc company. By doing so, UPS now owned the MBE corporation and then forced contractual obligations on the privately owned MBE franchisees. These franchisees changed their privately owned businesses name to 'The UPS Store' and the shipping rates were lowered to actual UPS rates. This is one detail of the pros and cons as to how this takeover affected the livelihood of the people that owned these now conveniently located 'UPS' locations. The revenue per transaction dropped, but increased customer count helped most locations break even as compared to the previous business model.
"What's your point?", you may be asking. Arguably, both MBE and UPS intentionally misled franchisees with false predictions of sales numbers and benefit programs to entice store owners to make the 'voluntary' switch (it was not voluntary). After the conversion, some struggling centers went under and revenue significantly dropped for many others. UPS customers now had convenience of location and service as well as low UPS rates at the expense of the owners of these facilities. Ironically, UPS corporation now became The UPS Stores biggest competitor. UPS corporate used its' power to gain customers by using advertisement and phone sales and directing customers to a convenient The UPS Store with prepaid labels. Though the rate is the same and often less at The UPS Store to pay for shipments, The UPS Store was now losing the profit portion it once generated. The proportion of these prepaid dropoffs compared to in center transactions had increased at an alarming rate. This, again, was causing many of these privately funded businesses to financially struggle. Since then, The UPS Store owners have banded to discuss such issues. Some filed a class action lawsuit. Others, more constuctively perhaps, worked with UPS to lower rates further and create monetary incentives which benefit The UPS Stores and are passed onto the customer.
So now, more to your point. There is a small handful of these business owners, concrete (non-abstract) personalities I would think, that see prepaid dropoffs as a threat to their livelihood. Obviously, it is entirely unfair and unprofessional to treat any customer with distaste simply because they are using that privately funded location for their convenience - while not providing any service fee to assist that business with the ability to financially continue offering the same services that these customers need. Many customers have no idea that The UPS Stores are privately owned and operated (yet still are in fact genuine UPS) and therefore should not be inflicted with the treatment such as you'd experienced. This particular owner made his personality shortcomings obvious, perhaps in the same manner as did your frustrations (sorry, I think I said I'd put that to rest.) I apologize for the terrible mistreatment that you had experienced. For myself, drop off packages are welcome as a necessary part for many businesses and, if managed properly, can create new and repeat customers that offer a cash flow required to maintain any business. The owner/manager at the location that you had used is assisting with his business deterioration on his own accord.
As for whether or not your package would have gone out that day, that would depend on whether it was an air shipment or Ground. With it being 6:30pm, as you'd mentioned, the Ground shipment pickup had already been completed for that day atleast an hour prior. The driver that you had seen was there for air shipments only. Why that wasn't explained to you was apparently nothing but rude. You deserved a simple and courteous explanation. If you had received a polite answer and, perhaps a breif explanation, that The UPS Store may have created a new customer via directly with your or word of mouth.
I would also like to thank you for not using that 'concrete thinking' by irrationally branding all The UPS Stores with such behavior. There are people of all natures throughout all businesses. One dud person shouldn't negate the genuine decency of others.
Best of luck to you.