We have already joined every general whitelist, met every major email provider's anti-spam requirements, and otherwise taken every step possible to minimize any factors that could result in our thank-you emails getting erroneously filtered as spam.
Once an email message leaves our server, it's ultimately out of our hands what happens to it at the receiving end, how the buyer's email provider chooses to filter spam, what the recipient's own email settings are -- some people even have their own email accounts set to allow emails only from addresses already listed their own, personal email address book or contact list (aka a "personal whitelist").
If there were any way to force emails to always be delivered to the recipient's inbox and never filtered out as spam, you can bet all the spammers would just be abusing that method anyway, so it'd be useless for legitimate email.
In cases where the buyer does not find a thank-you email in their inbox, you can ask them to check their spam/junk/bulk mail folder and be sure to mark it as Not Spam, which will move it to their inbox and allow such messages through in the future, and may also help train their email provider's spam filters to better-recognize us as a legitimate sender.