PayPal form creation - html

Question

I'm having some trouble with a form I'm trying to create in PayPal for a WordPress website. It's a form that allows for different types of registrations and quantities - both of these work. At the end, my client also wants an option to donate any amount - this is not working.
The code at the bottom is only adding the amount entered as a 'note' to paypal, not as an actual monetary value to the amount being paid. I've tried 'name="amount"' as well.
<table>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input name="business" type="hidden" value="EMAIL" />
<input name="cmd" type="hidden" value="_xclick" />
<input name="item_name" type="hidden" value="Camper Registration" />
<input name="currency_code" type="hidden" value="USD" />
<!-- Provide a dropdown menu option field with quantity. -->
<tr>
<td><input type="hidden" value="Quantity" />How Many Campers?</td>
<td><select name="quantity">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select></td></tr>
<!-- Provide a dropdown menu option field with prices. -->
<tr>
<td><input name="on1" type="hidden" value="Payment Type" />Payment Amount</td>
<td><select name="os1">
<option value="deposit">Nonrefundable Deposit $100</option>
<option value="Before1/1">(Before 1/1/15) $210</option>
<option value="Before4/1">(Before 4/1/15) $225</option>
<option value="After4/1">(After 4/1/15) $250</option>
</select></td></tr>
<!-- Specify the price that PayPal uses for each option. -->
<input name="option_index" type="hidden" value="1" />
<input name="option_select0" type="hidden" value="deposit" />
<input name="option_amount0" type="hidden" value="100" />
<input name="option_select1" type="hidden" value="Before1/1" />
<input name="option_amount1" type="hidden" value="210" />
<input name="option_select2" type="hidden" value="Before4/1" />
<input name="option_amount2" type="hidden" value="225" />
<input name="option_select3" type="hidden" value="After4/1" />
<input name="option_amount3" type="hidden" value="250" />
<!-- Provide place for other donations. -->
<tr>
<td>Other Amount:</td>
<input type="hidden" name="on0" value="Other Amount">
<td><input name="os0" type="text" value="0"></td></tr>
<!-- Provide place for notes. -->
<tr>
<td>Child's Name(s) and Comments:</td>
<input type="hidden" name="on3" value="Notes">
<td><textarea name="os3" rows="4" cols="50" wrap></textarea></td></tr>
</table>
<!-- Display the payment button. -->
<input alt="PayPal - The safer, easier way to pay online" name="submit" src="https://www.paypalobjects.com/en_US/i/btn/btn_paynow_LG.gif" type="image" />
<img src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" alt="" width="1" height="1" border="0" />
</form>
this is the code in question:
<tr>
<td>Other Amount:</td>
<input type="hidden" name="on0" value="Other Amount">
<td><input name="os0" type="text" value="0"></td></tr>