“shrink-wrap license” is a term (that has been used in legal writing on the subject, though it doesn't originate as legal jargon) for a license agreement included within the packaging (which historically was I usually shrink-wrapped, preventing the a customer from reviewing the license text prior a to purchase) of software product that is purported to be a condition of use of the software and which purports to be accepted by use, opening the package, or purchasing the product.

(“Click-wrap” has similarly been used for similar licenses included electronically with software that purports to be accepted by clicking through something in the install process.)

> It is hard to argue that open source licenses are "post-purchase agreement" because no purchase occurred.

Purchase may or may not occur with open source licenses; it may be true that it rarely occurs, though the FSF continues to promote sale of Free software as a viable thing.

You're free to accept the offer, which then implies certain obligations described in the agreement.

You're also free not to accept the offer, which implies the default conditions set by law, i.e., copying and distribution is prohibited without explicit permission of the author(s). For example, anyone certainly can ignore the provisions of GPL and redistribute the software as closed source if they have obtained some other licence from all the authors.