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What are the fees associated with a PayPal Buy Now button?
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The fees are

2.9% + $0.30 per transaction for all sales totalling less than $3000/month
Example:
$100 Sale: $2.90 + $0.30 = $3.20 fee = $96.80 deposited to your account

2.5% + $0.30 per transaction for all sales totalling $3000 – $10,000/mo
Example:
$100 Sale: $2.50 + $0.30 = $2.80 fee = $97.20 deposited to your account

2.2% + $0.30 per transaction for all sales totalling $10,000 – $100,000/mo
Example:
$100 Sale: $2.20 + $0.30 = $2.50 fee = $97.50 deposited to your account

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REF
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https://www.paypal.com
Business tab, Products & Services, Website Payments Standard, Pricing tab (lower down the page)

http://www.anysitesupport.com/paypal-buy-now-button-fees/