I was wondering if there is a reason to use shipping partner instead of invoice partner as the transaction partner.

In website_sale controller :

acquirer.button = payment_obj.render(
 cr, SUPERUSER_ID, acquirer.id,
 order.name,
 order.amount_total,
 order.pricelist_id.currency_id.id,
 partner_id=shipping_partner_id,
 tx_values={ 'return_url': '/shop/payment/validate', }, context=render_ctx)