I don’t know of such a service. Some sellers will ship 5.00-20.00 cards via forever global stamp which is about 1.20 just in an envelope…especially if you ask them. Since there’s no tracking tell them that if it gets lost you’ll assume the loss. That should hook many of them:)
I use Shipito and I’m satisfied ! (I live in France)
In Canada it is $2.50 for an international stamp, so it isn’t that bad?
In 18 years of buying stuff in the US, Japan and Australia I never used a middleman/shipping service.
Why? Because there was never any benefit on fees.
Shipping from EU to US is almost the same as US to EU.
With a handler you have to pay domestic fees, the handler wants a small fee for keeping/storing your mail, repackaging and the service he provides and you again pay fees to ship it over.
As @garyis2000 points out. Contact sellers before going into purchase. The $23 some ask for shipment is just a flat rate for anything to 4lbs that crosses the Atlantic. And since you buying singles that’s a total scam, unless you buying high value cards.
I see many eBay sellers offering shipping up to 8 single cards for under $5. But shipment can go as low as $1.75 when just using Economic shipping but takes longer and no tracking. But personally I take that risk if card(s) are below certain value.