You need to try Endcia if you have enough volume. USPS international shipping a simple and painless right from your nearest mail box. Print the label with the customs form and mail it.
The prices to Mexico and all international destinations have increased a bunch. I would estimate about $7 to $11 in postage if its under 12 ounces. Its First Class International Parcel now. I miss the previous $3 to $5 days of yore for small letter weight packages... My international customers dried up after the postage increase a bunch.