From a retial point of view, they work well. We allow customers to spend
less than the value of the certificate, reducing the value over a period of
time. This means they may only have a few dollars left at the end, which
will be credited towards a higher priced item. We record this on the
certificate itself, you may have to keep some other kind of records if you
chose to let customers do this. So far in all my years of retail and gallery
management, they have been a very good thing. A customer with a gift
certificate is usually happy from the start!

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Marisa (AU/NZ)
http://www.galleryvittoria.com
"She who dies with the biggest stash, wins."