A phytosanitary certificate is needed for shipping lilies from Canada into the US, so I imagine that one is needed for shipping from the US to Mexico. The provider of the plants is the one who gets the certificates. Smaller operations that sell to other countries as well as their own pass that price on to the consumer. (its usually $15). The large dutch exporters are also getting some version of a phytosanitary certificate for their exported plant material, you just weren't charged separately for it. Since they are exporting almost everything they grow that cost is built into their prices.

Canadian companies do a fair amount of their business with the US and so are willing to do phytosanitary certificates. They might be willing to ship to Mexico as well. The Lily Nook has a very nice selection of lilies. Unfortunately, they have already shut down their fall shipping.

I'm assuming (since you're a botanist) that you know that lily bulbs (with only a couple exceptions) need a cold vernalization period to come up in the spring. It would probably be easier for you to buy them pre-cooled in the spring. That would save you one winter of refrigeration.

-Helen