Possible to create two kinds of gift vouchers?

[schoolboy]

...we would like to be able to use gift certificates as a reward program...

You may be able to achieve what you want using Discount Coupons. I realise this does not ADD credit to a customer's account in the manner you are keen to have, but customers aren't dumb. They can do simple arithmetic. If I had been advised that I have a coupon worth $10 for your store (applied to specific products and possibly even subject to a date deadline), I would know immediately that if I shop for $100 worth of appropriate product, I'll only end up paying you $90.

The key is in the way you promote it to your customers.

[lyricwings]

Thanks, Schoolboy. I know that discount coupons work well for giveaways and such, but what I'm really after is the idea that every week, the amount in the customers' account grows, based on their participation. So that they keep getting an ongoing incentive, and any time they log into the store, they can see how much credit they have which they can exchange for goods, and it keeps growing every week.

To try to do this with coupons, I'd have to send someone a $5 coupon one week, and then send them a $3 coupon the next week, and then send them a $7 coupon the next week -- etc. But then they can't combine those to use for more expensive items, since only one coupon can be used at a time. And they can't ever check their "balance" to see how their credit increases with their increased participation in the community.

See what I'm getting at?

[schoolboy]

Hmmmm... this then looks like a "from the ground up" project. I don't know of any standard or 3rd party mod that achieves what you require.