Product page Alteration

I have an issue with my businesses shopify website that I am trying to figure out how to fix and was wondering if you would be able to help. We have a site to sell paper. And one of our many products is envelopes. Well, we created the product page. Added the variants but now we have to modify inventory for each of those variants. See, we sell bundles of 50, 250, & 500 at different costs depending on what bundles they would like to buy. It is all the same product but we have to update each variant with current stock. Is there a way to only have 1 inventory amount to update and when a customer purchases this, it deducts from only that one inventory amount and have the different prices when selecting that amount... if that makes sense. Or is there another way to design this so it makes sense for both the customer and us to manage the inventory level easier. We only need this alteration on specific products so it would only do this on the Envelope products.