Sure, create a local variable
localvariable = round(random(0,100))
Then make some subevents
localvariable < 5 -> drop item 1
localvariable < 40 -> drop item 2
explanation, random(0,100) will return a random number between 0 and 100 but it will have a decimal point attached (this is called a floating point precision number) so the call to round() makes it a whole number. You type this directly in as the expression in the "set variable" dialog.
The subevents are "if statements". If the local variable is less than 5, then drop that item.
if the local variable is higher 5, and lower than 40, drop that item (this gives a 35% chance, since 0-4 is already accounted for)