Flat rate for an *envelope* is $4.90 to $5.15 depending on the region. I don't ship in envelopes because there's a greater chance of the book being damaged. I ship boxed Priority which gives the comics the best chance to show up in the exact condition they were purchased in. It's also better for the customer as I sell many more higher grade books than I do lower grade.
Also to address an earlier point, a higher opening bid means a higher cost on my end to list the book in the first place. Multiply that by the 510 auctions I'm running this week and you've got a larger operating expense on my end even if it's 10 cents more to list each book.
Finally, a $6.50 shipping charge means I only have $5.85 of that available to me to pay for the cost of shipping the book. Minimum charge to me for shipping one comic from SC to CA is $6.51.
Customer service is the bottom line here while still keeping it realistic for my business to run smoothly.
Peace,
Chip