There are differences in plastics and differences in metals. The metals we cook with and use for eating utensils are deemed food safe..... that's why we use them. Other metals, just because they're metal, doesn't necessarily mean that they're meant to use with edibles.
Same with plastic. All plastic isn't food safe. Pvc isn't always food safe. The inside of PVC water pipes is safe... if they're the pipes that bring water in. The outside of the pipes and pipes for drains out may not be.
Thanks, Leah, for letting me know. I'd be hard pressed to order a cake from you