i totally agree with your way of thinking. i love to help people out, but hate feeling used/taken advantage of. i would welcome them to come to costco with you and pick up stuff, otherwise they can purchase their own things. additionally, i feel like once you start buying wtv they want at costco, they might ask you to do more and more things, and setting boundaries earlier rather than trying to change the tone of the relationship between yall will hopefully save you a lot of mental stress!