If you're collecting newsletter subscribers on your store's checkout page you'll want to ensure these new contacts are synced to Klaviyo instead of Jilt.
Disable Jilt's checkout sync
For WooCommerce stores, this setting is found in Jilt under Settings > Shop settings > Storefront settings. Remove the checkmark next to the "Show an opt in at checkout to ask for consent for marketing emails" setting.
Disable checkout opt-in on WooCommerce
For Shopify stores it's not necessary to make any changes in Jilt as Shopify has a built-in feature for capturing consent via their checkout page.
Enable Klaviyo's checkout sync
For WooCommerce stores, the checkout page settings are available in your WordPress admin at WooCommerce > Klaviyo.
They go into more detail on the checkout opt-in options in their setup guide here.
For Shopify stores, navigate to the Integrations page in your Klaviyo account and check the box to Collect email subscribers (optionally select a list from the dropdown).
Save your changes by clicking Update Shopify settings.
Continue to Step 5: Migrate autoresponders.