Just got a message from my customer that he had to pay an additional $4.40 at the USPS to get his package.

I sent one chip in a 4 x 8 bubble mailer. Paid for first class parcel rate with delivery confirmation at a cost of $1.75 and they tell my customer he had to pay $4.40 extra?

Anyone here have any idea what the extra fee of $4.40 is for? $6.15 to send one chip?