You might take a look at https://my.ecwid.com/cp/ They have a free version where you can have up to 100 products in your "store". You can upgrade for about $15 to add 10,000 SKUs. I'm not sure they can do what you want, but it's a very flexible shopping cart solution. Good luck, my friend!
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It's quite easy. I did it on my website here: http://smartwebmobile.com/make-payment/ Just create a Pay Now button without the amount entered. It will generate the button that your customer can fill in. Hope that helps.
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Can I create a Pay Now button that allows the customer to enter the amount to pay? This would be a button where the amount changes from customer to customer. It would represent the Balancer Due on an invoice that was created earlier and is different from every other invoice.
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