If I want to have an online store that takes credit cards, do I need a .com domain or can I use a .us or .org or whatever extension?
Domain name doesn’t limit payment processing.
I recommend you to use any payment processors like SWEREG, 2checkout or paypal if you like to process credit card payments without problems
You have a business website so .com is best otherwise .biz will be okey.
If business is country specific then TLD of that country will be fine.
As you want to develop an e-commerce website, I would suggest dot com extension suits your business requirement e.g. amazon dot com, ebay dot com etc.
You can get free domain name with reliable business class hosting package.
You can manage your EShop through an online control panel with FREE setup of online credit/debit card payments or payment gateway (PayPal etc.)
If you want to have your own secure server (for your domain), then all you require is a DEDICATED IP Address. Secure Server Certificates require that (and a bit of personal information).
I recommend you using .COM domain name if its availible. If not use -.Biz domain name, do not use .ORG domain name for your business web site.
You can use “whatever extension” - it makes no difference.
Also stay away from .me, .org, .tv, .tel if it’s going to be an online E-Commerce store.