Many email clients and hosts are increasingly sensitive to anything that could be spam. So they often reject any email from a new contact with an attachment, or that seems like an order. That reduces scam emails – but means if you do place an order then the automated emails and/or attachments may never arrive.
Generally, in most email programs, if you add the wanted email address to your contacts or address book that should do the trick. Some emails clients will have a whitelist option in security or privacy settings somewhere.
Below are some guides for a few popular email services – they may change their menus or settings at somepoint though – so you may need to look around a bit to find it if a recent update moved it.
The emails you will get will come from firstname.lastname@example.org
WikiHow has nice guides for adding to safe senders which includes screenimages.