is the repeating format of the URI.... using the word "product" for each item...
RewriteRule ^products/product\-([^/]) /products.php?product\-$1 [NC,L]
should result in /products/product-77a
IF it is something like products.php?shoes-66x, products.php?shirts-21b
RewriteRule ^products/([^/]+)\-([^/]) /products.php?$1\-$2 [NC,L]
should result in /products/shoes-66x and /products/shirts-21b
.... but only if I've already had enough coffee this morning!