The way the statement is written, your first else case is 0, when it needs to be the next if() statement. Basically, it reads as:
if culvert type is CSP
then int(${InvertSedimentRating}*2
else 0
----- the function stops here, because that is a full statement. Anything else after this in the same calculation either isn't evaluated or just plain breaks.
In order to nest your if() statements, the next if() needs to take the place of the preceding if()'s else case. Simply remove the "0)," from the first if() in your calculation, and close up the parentheses at the end:
if(selected(${CulvertType},'C.S.P'),int(${InvertSedimentRating}*2),if(selected(${CulvertType},'H.D.P.E'),int(${InvertSedimentRating}*3.2),0))
if culvert type is CSP
then int(${InvertSedimentRating}*2
else if culvert type is HDPE
then int(${InvertSedimentRating}*3.2
else 0