[mothered]

Did you actually box them?

I sealed the box with white tape, then cut it open and closed it again with green tape

That's boxing them.

(this was visible on the pictures they provided) but apparently this wasn't enough because they claimed there were no signs of tampering.

I pointed out the fact that there was tape on the box I didn't use but they ignored me.

Ask them to provide evidence that every package they tape and dispatch, Is In the same form of taping (colour-wise) as the one you returned.