Here's the problem:
Amazon payments charges:
Transactions greater than or equal to $10.00: 2.9% + $0.30 per transaction. So a $25 donation yields $23.97 to archive.org, or 95 cents on the dollar.
Transactions less than or equal to $9.99: 5.0% + $0.05 per transaction. So a $5 donation yields $4.70, or 94 cents on the dollar.
Now, they are most likely using Amazon's volume payment system, so are paying between 2.2 and 2.5% + $0.30 per transaction,
but only for transactions greater than $9.99. Let's assume the lower rate (although it's calculated as a 3-month average, so I doubt they're eligible). Now a $25 donation yields $24.15, or 96 cents on the dollar. It's just not as cost-effective for them to solicit tiny donations (btw, PayPal's rates are similar).
As to the worth of the service, whether you use it or not, archive.org is still there 24/7/365. Would you prefer to submit your query and then wait 72 hours for the results?
..and yes, I donated also.