How to multiply drop down menu times product price on sales sheet

I'm not advanced at all in VBA, or Macros for that matter, and may need this explained to me like a 5 year old, if possible.

I am creating a sales sheet to send to customers, that can be filled out by them and have the total calculated. I have attached what I am thinking for the spreadsheet. However, I don't even know how to populate the drop down menu.

We sell our products wholesale in multiples of 20. I need to know how I can have the drop down menu options be '20, 40, 60, 80, & 100', which then multiply by our $3.50 cost, at which point can have the $5.00 added whether or not the display option is chosen..displaying the total amount in the far right column.

I hope this is not too confusing..please let me know if it is so that I can better explain. Thank you for any and all help offered.