If you're in the US, then you could use a P.O. Box OR you could USPS general delivery. Just be aware that a) the package may go to a different post office than the one specified (typically there is a dedicated post office for a given region), b) there are specific requirements for the formatting of the address, c) some post offices may be weird about it (due to volume and/or not wanting you to use it as in place of a P.O. Box since it is intended for those without a physical address, chiefly vagabonds and the homeless) and/or not let you send more than a couple of packages to the same post office, so call and ask to speak with the postmaster beforehand if possible.