Simple use of basket custom fields to save the shipping selection if the customer decides to edit their order from the OPAY screen. Utterly useless in my opinion but a client requested it so here's the code.

OPAY

<mvt:comment>
#
# the following two lines are stock miva smt to save order instructions, so let's add our shipping method save under this
#
</mvt:comment>
<mvt:item name="customfields" param="Write_Basket('order_instructions', g.order_instructions)"/>
<mvt:item name="customfields" param="Read_Basket('order_instructions', g.order_instructions)"/>

<mvt:comment>
#
# save our shipping method to a basket field so we can auto select it in case they edit their order from OPAY
#
</mvt:comment>

<mvt:item name="customfields" param="Write_Basket('ShippingMethod', g.ShippingMethod)"/>
<mvt:item name="customfields" param="Read_Basket('ShippingMethod', g._ShippingMethod)"/>

OSEL

First add the "read" basket field above the shipping methods form:

<mvt:comment>
#
# we saved g.ShippingMethod as a custom basket field on OPAY. If they edit their cart info this should give them the same
# shipping method as previously selected. We read into g._ShippingMethod instead of g.ShippingMethod so we don't confuse miva merchant.
#
</mvt:comment>

<mvt:item name="customfields" param="Read_Basket('ShippingMethod', g._ShippingMethod)" />

<form class="t-osel-form" method="post" action="&mvte:urls:OPAY:secure;">

Next, find the loop and update the l.pos1 condition to include whether or not we have g._ShippingMethod

<mvt:if expr="pos1 EQ 1 AND NOT g._shippingmethod">

Finally, after the mvt:else for the condition above, we wrap the radio input in a condition that tests for g._ShippingMethod

<mvt:if expr="g._shippingmethod EQ l.settings:method:module >
	<input class="c-form-checkbox__input" type="radio" name="ShippingMethod" value="&mvte:method:module;:[sbmvte:method:code]" checked>
<mvt:else>
	<input class="c-form-checkbox__input" type="radio" name="ShippingMethod" value="&mvte:method:module;:&mvte:method:code;">
</mvt:if>