having trouble with nested IF function

=IF(Amount_Purchasing>0,Current_Processing_Fee,IF(Amount_Purchasing>=10,Current_Processing_Fee+Extra_Handling,"-"))

I am creating a buy sell ticket for a school project and this is supposed to have a processing fee of $2.00 (Current_Processing_Fee) unless the user enters an amount of 10 or above in the Amount_Purchasing column, in which case it should ADD Current_Processing_Fee+Extra_Handling however it isn't. I continue to get the $2.00 original amount.

I have also tried the following format with the same results:

=IF(Amount_Purchasing>0,Current_Processing_Fee,IF(Amount_Purchasing>=10,Processing_Fee=Current_Processing_Fee+Extra_Handling,"-"))