Is anyone aware if I can use media mail to send books internationally? I have books I would like to send to someone in Spain, but I’m worried about costs of shipping.
No, media mail (the US version, anyway) is domestic only - though it does include far-flung US locations such as military APO addresses.
There used to be a slow-but-cheap international rate, but it went away some time ago. I've had to cut back on international mailings since it can cost as much to send one paperback internationally as it does to mail a 20-pound box of books domestically!
There is the M-bag option, which is a kind of bulk rate; even that is pricier than it used to be, and has its own set of requirements, but if you had a number of books to send to a single address you could look into it.