I really like the idea, and would love to get my hands on the meter once I get around to getting a membership.
Personally, I see no harm in there being a $5 charge above the price of shipping. One, it allows the risk to be minimized, as well as generating a bit of income for you/ other group buys for the benefit of TPT community. I would gladly pay up $5 extra myself.
What I would really like to see if this comes to fruition is a collection of data about the various common light fixtures that we see in the hobby today. We would need to come up with a few standard measurements locations that should be taken for every light. However, I believe this type of information would easily become indispensible in making good purchases of lighting fixtures in the future.
Think about it. You want to figure out if you can grow plant X with light Y. This is an exceedingly common question on the lighting sub-section of the forums.
If we get this data together, it will be easy enough to know the answer for your question without a large search or posting a new topic. Simply look up how much PAR your proposed light fixture outputs at your predetermined height of the light above the substrate. Factor in a couple other variables such as substrate, CO2 and fertilizer dosing regimen (or lack thereof) and your question is answered.
In my eyes at least, this has the potential to be something much greater than simply sharing an item normally not available to your typical hobbyist, a chance to really get some significant research done for the benefit of the community that we can all use.
This is also where I like the idea of including a $5 usage charge atop the price of the shipping cost. If we wanted to, another meter could be bought, effectivly doubling the rate at which data could be gained. I agree that it also makes sense to only allow people who are supporting the website to allow the meter. Not to say that it is true in all cases, but you can generalize that those people who support the site are more serious about the hobby as a whole, and therefore more likely to respect this service as well as more likely to provide good data readings on their lights.