Well first off it appears to be able to all handled with just standard html.
Try changing:
<tr>
<td>Amount:<!-- //amount shown on page and set amount --></td>
<td>€
<input type="text" value="00.00" name="amounttoshow"
id="amounttoshow" style="border: none; text-decoration: none; font-size: 15px;" disabled>
<input type="hidden" value="00.00" name="amount"
id="amount" style="border: none; text-decoration: none; font-size: 15px;" >
</td>
</tr>
to:
<tr>
<td>Amount:<!-- //amount shown on page and set amount --></td>
<td>€
<select name="amount">
<option value="277.00">TRIPLE OCCUPANCY: € 277.00 per person</option>
<option value="325.00">DOUBLE OCCUPANCY: € 325.00 per person</option>
<option value="497.00">SINGLE OCCUPANCY: € 497.00 per person</option>
</select>
</td>
</tr>
that may all that you have to do.
post back if that does not do it. i have worked with payment systems before and they all have they're own underlying commands, so it may not work. once again
somebody may have to pour / study the existing code to see what was being done and how it was being done