Ray, changing the shipping to "Next Day Air Saver" service did the trick by charging the correct shipping amount. Thank you.
As for the advantage of separate carts for subscriptions, I will have to think the whole concept through more thoroughly. The reason that I brought up separate carts is because in doing some research on websites that sell both subscription and non-subscription type products, some websites "wipeout" the contents of subscription products when a customer adds a subscription product to their shopping cart. You can checkout harrys.com to see how they approach subscriptions during checkout. I think that this was done to avoid problems with shipping charges.
In retrospect, I probably think that anyone who would want to purchase a subscription on my website would most likely not be mixing subscription and non-subscription products during the same checkout period but I cannot be 100% certain of that for every customer. I am still a little ways off from finishing my website, including the subscription service. I am sure that I will be in touch when I inevitably get stuck during the subscription programming process. I may even need a premium ticket if I get really stuck. Once again, thank you for all of your continued help.