Tom, we go in and out with sets of samples or certain tools all the time.
We usually have a commercial invoice detailing the items - saying clearly not for resale and showing a value of 0. This is usually sufficient if we get asked at the luggage inspection.
If this is clearly not sufficient to satisfy the officials, ask to speak to the head of customs of the airport that you are flying into. Explain that the items are to be used and taken out of the country a few days later. The head of customs may want you to temporarily import the items and will decide on an amount. Pay this and you have your stuff. Ask them how to claim your money when you leave. They probably don't know. When you leave, go to the airport an hour or two earlier to try and get your money back. You may, or may not get it back. Consider this the cost of doing business as the process to claim back, may not be worth the trouble for a few dollars.
You may be really lucky and walk through without a customs inspection. This has happened to my husband and some sales people a few times at POP airport.
It will be helpful if you have someone with you that speaks Spanish.
The absolute best way is to carry it with you in a computer like back-pack. Often they don't even glance at what is on your shoulders.
