The syntax is listed in the "Scripting Reference" under LiveCycle Designer's menu "Help" option.
if ( simple expression ) then
list of expressions
elseif ( simple expression ) then
list of expressions
else
list of expressions
endif
There are also examples available.
You could also use JavaScript and the "switch{case():...}" statement. Wich might more clearly state and test the comparisons.
What is the shipping charge if the Subtotal is $25.00, $25.01, $50.00 or $50.01? You have to be careful with the end points of ranges.