There are ZERO storage fees for cards that are not for sale.
There is a 1 cent per month storage fee charged per card that is listed over 75 cents, charged for whatever is listed at 12:01 PT on the first of the month. If you are the cheapest person on the planet (ME), you go in on the last day of the month, unlist them, and relist them again the next morning on the first. And yes, easy enough to do when it's only 2 cards. Not sure it would be worth it if I had 1,000s there.
Shipping promotions happen when they want, but I only know of Black Friday time frame that has happened each of the last 4 years. I know they did one after the restructure as a thank you to those who didn't bail on them.
And yes, do the challenges when you can, get points, get free COMC credit. If you plan to spend money on there, it's free money. I have gotten over $90 in free money from them over the last couple years. (I even got the little blue shield next to my user name. :) ) Again, being cheap, I'll take every free $ I can.
Also, when you carry a monetary balance, you can make offers for cards on there. Sellers can set a tolerance where they will accept offers to a certain percentage. Sellers can accept, counter-offer, or decline. I wish decline wasn't an option. If you don't like my offer, let me know what price you can do. Now that I've actually tried to sell on there, I understand how it works from the seller's side, and why they may not accept the lowest possible offer. But it certainly doesn't hurt to ask.
If you have any questions on the buying or selling process, feel free to ask here. You have a lot of people with knowledge from both sides who are willing to help out.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).