You could simply create and send and email invoice to each client (no PayPal Account required for your clients to pay) or as an alternative if you have a web site you could create a Buy Now type item button (no PayPal Account required for your clients to pay) and tweak the code a little so the client can enter the amount on your web page - pay your bill online - for options you could include a text input field for their name or invoice number or customer account number or whatever info you need. You can also do more advanced things and use a script to make the input fields required but, that's another topic. For learning purposes, below is an example of how a basic pay your bill online button would work.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Buy Now Button Example</title>
<!-- START META TAG SECTION -->
<meta charset="utf-8">
<meta name="description" content="">
<meta name="keywords" content="">
<!-- END META TAG SECTION -->
</head>
<body>
<!-- START SAMPLE CODE SECTION -->
<!-- PARAGRAPH 1 -->
Pay Your Bill Online Examples
<br><br>
Buy Now - Customer Enters Amount, very simple.
<br><br>
<hr align="left" width="50%" noshade>
<br><br>
<!-- START CONTENTS -->
<!-- START BUTTON EXAMPLES -->
<form target="_self" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="on0" value="Customer Number">Please Enter Your Customer Number:
(Example: 99999999)
<br><br>
<input type="text" name="os0">
<br><br>
<input type="hidden" name="on1" value="Invoice Number">Please Enter Your Invoice Number:
(Example: 99999999)
<br><br>
<input type="text" name="os1">
<br><br>
Enter Amount Due :
(Example: nnn.nn)
<br><br>
<input type="text" name="amount">
<br><br><br>
<input type="reset" name="reset" value="Clear Selections">
<br><br>
<input type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_paynowCC_LG.gif" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
<input type="hidden" name="cmd" value="_xclick">
<!-- Replace "business" value with your PayPal Email Address or your Merchant Account ID -->
<input type="hidden" name="business" value="your PayPal Email Address or your Merchant Account ID">
<input type="hidden" name="item_name" value="Online Payment">
<input type="hidden" name="item_number" value="OLP-00001">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="shipping" value="0.00">
<input type="hidden" name="tax" value="0.00">
<input type="hidden" name="button_subtype" value="services">
<input type="hidden" name="no_note" value="0">
<input type="hidden" name="country" value="US">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-BuyNowBF:btn_paynowCC_LG.gif:NonHosted">
</form>
<!-- END BUTTON EXAMPLES -->
<!-- END SAMPLE CODE SECTION -->
<br><br><br><br>
<hr align="left" width="50%" noshade>
<br><br>
NOTES:
<br>
In order to test the code, you must replace the "business" value variable with your PayPal Email Address or your Merchant Account ID.
</body>
</html>
