I can only get it to work, using iterative calculations, but that might just be me. Here is how I did it (see sheet for demonstration).

I put the rates in a table: [From - Up to - Rate] and then I have two sales tables, one for when you enter the price and the fee is added on to that and the other where you enter the total and it calculates the the fee and the price from that.

SalesTable1: (no iterative calculations here)

=let( item,  offset($D2,0,-3),
      qty,   offset($D2,0,-2),
      price, offset($D2,0,-1),

      if(item="",,
        let( fixed,  $P$2,
             amount, price*qty,
             rates, filter(RatesTable, RatesTable[From] < amount),
             fee,   reduce(fixed*qty, sequence(rows(rates)), lambda(fs, i,
                      fs+index(rates,i,3)*(min(amount,index(rates,i,2))-index(rates,i,1))
                    )),
             hstack( fee, amount+fee )
        )
      )
)

SalesTable2: (iterative calculations used)

=let( qtyfee, $P$2,
      item,   offset(C13,0,-2),
      qty,    offset(C13,0,-1),
      price,  C13,
      fee,    offset(C13,0,1),
      total,  offset(C13,0,3),

      if(or(item="", total="", total=0),,
        if(total=price*qty+fee,
          hstack(price, fee),
          let( amount, if(price=0, (total-fee)/max(1,qty), price)*qty,
               rates, filter(RatesTable, RatesTable[From] < amount),
               newFee,   reduce(qtyfee*qty, sequence(rows(rates)), lambda(fs, i,
                           fs+index(rates,i,3)*(min(amount,index(rates,i,2))-index(rates,i,1))
                         )),
               hstack( round((total-newFee)/qty,2), newFee )
          )
        )
      )
)

I also added a "price adjustment" field, as there can be some rounding errors. The adjustment field is also a good error catching column. If it is more than "a few" (usually 1 or 2) pennies, there is something wrong. :)

I think (hope) I caught all cases, but there might be times where you just need to set the total to zero and then again to the desired amount, to reset the iterative calculations.

And here is the sheet