just lower the handling charge accordingly taken the tax rate you set into account i would have thought

example, if your tax is 10% and you want to have 2% handling charge but without applying it to the tax, set the handling charge to 1.81818%

i.e (2/110)+(2-10%)

> I get 1.92 if I do the equation with adding the two

then you need to buy yourself a new calculator
2/110 = 0.018181818…
2-10% = 1.8

which clearly sums up to 1.81818….

strangely, that calculation – with your numbers (6% / 15%) is not accurate
it returns 14.241509433…
when it should be 14.150943 (you’ll still get some rounding errors if a sale is over 1,000,000 but i’m sure that’s good enough for this excercise)

just enter this (i.e 14.150943) into the surcharges percentage field while i figure out why this is different (kind of for my own entertainment and future reference)

ok, calculation should actually be like this (just so happens that using 2% and 10% in my initial example produces the same result …..lol)

x = vat/sales tax
y = handling charge
calculation should be

y – (y/10x*x)

so – in your case:
15 – (15/106*6) = 14.1509433…

PS: in case someone else reads this.
percentages should be entered with either upto 2 or 4 (or more) decimals otherwise you will end up with a number in the thousands (i.e 14.123% would be saved as 14123%) that’s because of formatting sanitisation and the fact that there are actually some currencies that use 3 decimals (for reasons i do not know)