She just didn’t get it

Went to the post office yesterday. The missy told me it was $380 to mail the letter I had. She gave me 3 x $60 and the rest in $40 stamps. So I asked her if she had run out of $60 stamps. She asked me why. I told her she could have given me more $60 stamps so I would have less number of stamps. She said she didn’t have any $100 stamps, so the 3 x $60 was to make up $180 blah blah blah.

Why didn’t she give me 6 x $60 and 1 x $20? That was the point I was trying to get across but she just didn’t get it.