I haven't read your description of the problem fully but you can replace your current formula which is incorrect with this

=IF(A20="";"";VLOOKUP(A20;'"Supplier Data Sheet'!$A$3:$Q$100;LOOKUP(E20;{-1E99;91;100;126;168;300;400;500;2000;3500};{3;15;9;16;17;10;11;12;13;14})))