[mothered]

Since you have made a change earlier, you should know how the process works. 

They're attempting to put you at fault, so reverse It.

Tell them that It's not a matter of knowing how their process operates, but rather their neglect to provide shipping Instructions that ultimately resulted In sending the Item via the said carrier. Assure them that If they specified how the goods should be returned, you would've complied with their request. However, they didn't, so you've done the right thing by sending the Item back as per their Instructions.

If you have a picture of the package and can show what it looks like it would be helpful.

If that's what they want, that's what they'll get.

Make up a package at home, take a photo and send It to them.