DIFFERENCE IN POSTAGE CREDITED TO YOU
By mistake, we shipped your order (#[ORDER NUMBER]) [SHIPMENT TYPE, ex. Overnight] instead of [SHIPMENT TYPE, ex. Express] because we thought time was of key consideration. Since this is our mistake, [we have credited the difference to your account / we have enclosed a check in the amount of the difference].
Please do not hesitate to call me at XXX-XX-XX if there is something I can do for you.