Post Office Ltd

Posted parcel, a box containing shoes, postage cost £6.57, following week posted back to company exact same box containing exact same item, untouched other than to enclose return form in both boxes, these boxes were identical in every way, including the contents, that were sent in error, however on posting back the second box , low and behold the postage jumped up to £10, they tried to say one box was a little longer than the other, I had these boxes at home one on top of the other, exact same size, I am 76 and live only on a pension , I do not expect to be ripped off by Royal Mail, they even mentioned it may have had an extra piece of sellotape, Hmm! very heavy sellotape, there were no sellotape added to these boxes, not good service at all