Don here from Shopify. :)
I'm not familiar with Venmo myself though a quick search online informs me that this is a mobile payment service owned by PayPal.
Checking out your store it looks like your prices are in USD so I would presume from that that your store is based in the USA and as such can use any of the payment gateways available for use in the USA with your Shopify store.
While I've not heard of it being used on a store here, you can still reach out to PayPal support to see if their gateway integrations here provide this feature or if they would be willing to work towards providing this in future.
All the best!