It sounds like you need to invest in an online ecommerce portal. There are several companies that host these for you and provide good infrastructures. There are a lot of great third party ecommerce portals that offer all of these features (searchable database of products each with SKU's auto generated or assigned and PO's, invoice features, sales tax computed based on state, not sure about international, and payment gateway integrations with places like PayPal and CC processors like Authorize.net).

Here's one I found just by googling:
http://www.siteground.com/zen-cart-hosting.htm

You would just need to pick an ecommerce host like siteground and find what ecommerce carts they support (like Zen Cart), find the one you like and then there you go (usually they are turn key and you just need to spend time doing data entry).

Now those are mostly designed as store fronts, and it sounds like what you're looking for is more like an accounting system where a single accountant is managing the sales. For that I'd recommend something like Peachtree. There are various flavors.

You can find more info here:
http://www.peachtree.com/productsServices/compare/

What industry are you in? I ran a couple of hair salons and I was looking for something similar to what you were looking for (database, pos, etc.) I don't know what industry you are in but I'm sure you can find software for the particular industry you are in. Also, Quickbooks for Mac is a more generic application you can format to your industry. One last thing, again depending on your industry, their are a number of websites you could own for marketing and customer management. Google your industry plus "website builder". Hope this helps.