I'll check once mine arrives in mempools to see how the fees compare. I tried a few different amounts (large and small) and the fee was different and always close to 0.1% (closer the lower you set the on-chain fee). You can select the sats/vbyte for the onchain fee and I could tell that a fee of 0 sats/vbyte still left the lower bound of a 0.1% difference unexplained for