well put it this way. when u go overseas and make the purchase and charge it your credit card, regardless it is visa/master/amex/diners/jcb, they would make a margin based on the current market rate. however, you would not know the rate until you receive your bank statement.
paypal would apply more or less the same method. they would charge a margin based on the current market rate. the rate is more or less similiar to credit card co. rate. Difference is that, you gotto see the exchange rate at paypal.com
if you like the rate, you may proceed. if u dun like you make do the USDSGD conversion with the credit card co. Again, those rate might not be any better and yet, not transparent to you until you get the statement.
if you verify your acct, you will have more functions with your acc. I am not too sure what is your limitation of having unverified acct.