You have to download an older version of the electrum wallet, in order to get a 111 characters master public key. you can use i.e. electrum wallet version 1.9.8 (i dont know if that can cause problems with your current electrum wallet file, so i would install it on another computer or in a virtual machine.). Then you create a new wallet for your wordpress shop and export a new 111 characters master key.
Maybe you can restore your current wallet data with your wallet seed within the 1.9.8 .. but i dont know, i never tried that.
