Buy All - Minimum Quantity Or Purchase Amount

Is there any way you can think of to enforce a minimum purchase on a Buy-All? Whether that's a minimum quantity somewhere, or a minimum dollar value on the purchase?

I can't set the entire price list to a minimum purchase since there are some lower-priced items they can buy individually. I'm hoping to say they can buy all photos/favourites for 99¢/image as long as they purchase more than $20 worth.

(It's a charity event so don't judge the cheap pricing!)

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You can try to use the tiered pricing so when it is more than 20 photos they are .99 each
http://www.picturespro.com/sytist-manual/photo-products/buy-alls/

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So,
Quantity 1-1: $20.00
Quantity 2-2: $10.00
Quantity 3-3: $6.66
Quantity 4-4: $5.00
Quantity 5-5: $4.00
Quantity 6-6: $3.33
...
Quantity 19-19: $1.05
Quantity 20-0: $0.99

That's possible but a bit tedious. That's the only method currently available then?

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That is the only way I can think of doing it.

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