It may sound strange, but I found my local grocery store to have cheaper envelopes and index cards when I was looking for them.
The big office supply stores wanted some $2.19 (give or take a dime) for a 100pk of white 3x5" index cards, granted you can find plain white ones that are plain, white, and not part of some goofy tear out binder. Grocery store: $1.29. Same thing with a 50pk of security envelopes w/self adhesive flaps. Staples had their store brand for about $2.49, OM "Columbian" brand $2.19, grocery store "Mead" $1.89.
I'm not saying the office stores will always be more expensive but it does pay to shop around if you don't need it right away. Plus your local grocery store might not carry your favorite brand and type of pens. 😉
edit: you said website. oops.