Hello Andrew,

Yes, you can show payment options as checkbox option, but for this you need to override the template.
Refer this link for more information.
https://stackideas.com/docs/payplans/administrators/customizations/template-override

File Path : root/components/com_payplans/templates/default/invoice/default_confirm_details.php
Or if you are using bs3 template then
File Path : root/components/com_payplans/templates/tmpl_bs3/invoice/default_confirm_details.php

Here you can find the code near line no. 48

<div class="span6">

				<span class="pp-payment-method">

					<?php if(!($invoice->isRecurring()) && (floatval(0) == floatval($total))):?>

							<?php  echo XiText::_('COM_PAYPLANS_ORDER_NO_PAYMENT_METHOD_NEEDED')?>

					<?php else : ?>

							<?php if(count($payment_apps) > 1):?>

								<?php echo PayplansHtml::_('select.genericlist', $payment_apps, 'app_id', 'class="input-medium"' , 'id', 'title');?>

							<?php else : 

								$gateway = array_shift($payment_apps);

								echo $gateway['title'].'<input type="hidden" name="app_id" value="'.$gateway['id'].'">';?>

							<?php endif;?>

					<?php endif;?>

				</span>	

			</div>

Change this code as per your need.

Let me know if you have any query.