Signing documents for bosses okay?

for the past 3 months, I have been working as a receptionist for a biotech coy in TX that ships genes worldwide. They want me to sign the FedEx/DHL/UPS commercial invoices (name, title, signature and date) for international exports but I refused. I told them (albeit jokingly) that I cannot vouch for the contents of the vials and that these invoices are legal customs declarations. As an alternative, they proposed my signing *their* names on these invoices. They say that it is inefficient for them to be signing when they could be doing their work; besides, secretaries in other companies routinely sign on behalf of their bosses. I am concerned that if I do this, they will eventually offload other documents (NDAs, other contract, &c) on me to sign. what's your advice? thanks in advance!