Crystal Reports multiple variations in an If statement

Message

     I am trying to create a formula to set Sales tiers in a report based on a group sum field. 
if Sum ({oelinhst_sql.sls_amt}, {oehdrhst_sql.ship_to_name})>1000 then "Tier A" or
if Sum ({oelinhst_sql.sls_amt}, {oehdrhst_sql.ship_to_name})<=1000 and Sum ({oelinhst_sql.sls_amt}, {oehdrhst_sql.ship_to_name})>500 then "Tier B" or
if Sum ({oelinhst_sql.sls_amt}, {oehdrhst_sql.ship_to_name})<500 then "Tier C" else "Error"

I keep getting various errors with this formula. 

Can someone help me with this formula?

thanks