Most likely I would suspect the issue with email to Comcast is simply a matter of spam filtering and/or delivery delays at their end. We process orders, including sending thank-you emails, immediately when we get confirmation of completed payment, but once the receiving mailserver accepts the message, ultimate delivery to the recipient's actual inbox is beyond our control or insight. Delivery delays of up to an hour or more to major email providers are not uncommon, and this may affect some messages but not others depending on which inbound mailserver each message happens to hit.
That said, I'll refer this to Operations for further investigation. Have you been seeing this issue more or less often since Sept. 23rd, when we finalized migration of our services into the Amazon cloud? Since then, your Transaction Log shows about 3% of your orders have been from buyers @comcast.net; of those Comcast buyers, about 85% were able to complete their download in only 1 or 2 Attempts, and I'm seeing only one Comcast buyer who did not make any Attempts at all on their original link, though you did send them a free replacement link, on which they made 1 Attempt.
I would expect that Amazon SES is already registered for the Comcast Feedback Loop, which should cover that angle as the messages are actually being sent by their servers rather than yours or ours, and they would forward us any feedback regarding mail sent as From: our domain. Note this only pertains to email the recipient deliberately marks as spam, which is unlikely for emails delivering a link they'd paid for, and this feedback is really only used to keep complaint rates down by seeing which kinds of message are most likely to get marked as spam. Speaking of, for this reason and due to automated spam filtering, we recommend keeping your thank-you emails as brief and succinct as possible, avoiding unnecessary link URLs and promotional language that could trigger spam filters or a user's perception of the message as spam.
The Amazon SES Mail Simulator would not be useful to test messages to comcast.net; it only provides specific test addresses @simulator.amazonses.com that messages can be sent to, each of which respond to incoming messages in a predetermined way (e.g., accepts the message, or bounces it as undeliverable, or autoresponds, or marks it as spam, etc.), so the sender can see what happens in each of those scenarios.