If they'll only be ordering four items, you can easily just have one page which does that, then submits to another page which calculates the charge and auto-submits to Authorize.net (onload="document.forms[0].submit();" in your <body> tag). See Chapter 5: Additional API Fields for sub-section Itemized Order Information for how to specify line items in the form you submit to Authorize.net:
http://www.authorize.net/support/SIM_guide.pdf
<INPUT TYPE="HIDDEN" name="x_line_item" VALUE="item1<|>golf balls<|><|>2<|>18.95<|>Y">
<INPUT TYPE="HIDDEN" name="x_line_item" VALUE="item2<|>golf bag<|>Wilson golf carry bag, red<|>1<|>39.99<|>Y">
<INPUT TYPE="HIDDEN" name="x_line_item" VALUE="item3<|>book<|>Golf for Dummies<|>1<|>21.99<|>Y">
(yes, you put in one tag per item and they're all named the same...)
