Don Martín Enríquez, the viceroy in Mexico, wrote the King in December 1570, just after the first round trips across the Pacific:
And one of the difficulties consequent upon this commerce and intercourse is, that neither from this land nor from España, so far as can now be learned, can anything be exported thither which they do not already possess. They have an abundance of silks, and linen likewise, according to report. Cloths, on account of the heat prevalent in the country, they neither use nor value. Sugar exists in great abundance. Wax, drugs, and cotton are super-abundant in the islands, whither the Chinese go to obtain them by barter. And thus, to make a long matter short, the commerce with that land must be carried on with silver, which they value above all other things; and I am uncertain whether your Majesty will consent to this on account of having to send it to a foreign kingdom. I beg your Majesty to consider all these matters, to inform me concerning them, and to give explicit orders to the person in charge here so that no mistakes may be made.