Modifying that to accept SBD should be like a 2m job ... it's not exactly clear to me where the variable for STEEM stops, but show it to 
You just need to duplicate the 'steem' variable for 'sbd', change the URL to get SBD price instead of STEEM and then change this part
sbdTotal = total;
to
sbdTotal = total / sbd
as per the STEEM price formula.
RE: I Built an Online Store that Accepts Steem and Bitcoin as Payment!