You may want to consider setting up a table and using Vlookup. It's much easier to edit and friendlier to use.
I see the problem with your IF formula. The last IF needs to read IF(J2=10,o28*1.62,xxx)))
In other words, If J2 doesn't equal 10, it needs one last condition(xxx), such as IF(J2=10,O28*1.62,"Number not in list")))
so it can conclude.
Consider setting a table and using Vlookup. It would be much easier than an IF formula, and you could make the entry cell a data validation drop down list box.