Someone please explain the logic to me

Today I took 3 shirts into the dry cleaners

The lady was welcoming, pleasant, had a smile in all that.

 

She asked if Wednesday afternoon was ok to pick up, I agreed that would be fine.

She rang up 3 shirts at 3 x $5 = $15

Then she asks if I would like to pay now or on pick up

I decided to pay now, get it sorted

And in agreeing to pay upfront I received a 10% discount.

 

WHY? 

It's not like I wouldn't have paid full at whatever point they had asked

So why discount at that point just for paying in full in advance? Seems to me like you just lost out on a $1.50 of revenue?