So you need to sum the column, sort the data set, then calculate the cumulative percent. Use a custom format to create your Bucket column.
data sales;
input Customer Sales;
datalines;
1 15
2 14
3 13
4 11
5 12
6 18
7 21
;
run;
proc sort data=sales;
by descending sales ;
run;
proc sql noprint;
select sum(sales) format=best32. into :s from sales;
quit;
proc format;
value pctSales
0-.33='High'
.33-.67='Medium'
.67-1='Low';
run;
data sales;
set sales;
retain total 0;
format pctSales percent8.2;
total = total + sales;
pctSales = total/&s;
bucket = put(pctSales,pctSales.);
drop total;
run;