Code:
{* depends on your XC/smarty versions you may need to use back ticks around value; 7 * 24 * 60 * 60 = 604800 for 7 days, adjust if less or more days needed *}
{assign var="future_date" value=$order.date+604800}
{* make sure variable is actually $products and not $order.products *}
{* are you sure "amount" is price and not quantity? *}
{* replace yourdomain with your site's address *}
<iframe id="FSI_IFrame" style="display: none" width="0" height="0" src="https://api.feefo.com/api/sales/merchant?merchantidentifier=identifier"
data-feefo-email="{$order.email}"
data-feefo-orderref="#{$order.orderid}"
data-feefo-name="{$order.firstname} {$order.lastname}"
data-feefo-customerref="#{$order.orderid}"
data-feefo-feedbackdate="{$future_date|date_format:$config.Appearance.datetime_format}"
data-feefo-products=[
{foreach from=$products item=product name=products}
{ldelim}
"description":"{$product.product}",
"productsearchcode":"{$product.productcode}",
"amount":"{$product.display_price}",
"productlink":"https://yourdomain/product.php?product_id={$product.productid}",
"currency":"GBP",
"tags":{ldelim}
"saleschannel":"Web",
"productline":"games"
{rdelim}
{rdelim}
{if not $smarty.foreach.products.last},{/if}
{/foreach}
]>
</iframe>
{* check all lines for extra spaces - the forum tends to add spaces here and there *}