About what I figured. Pretty sure it is no longer possible to not specify at least one shipping option in a listing on ebay.com, so guessing it's a Smartpost listing issue.
There was (and probably still is) an eBay bug feature where certain non-machinable FedEx Smartpost errors cause that error message. And probable that for that printer SmartPost is the only shipping option on that listing.
A Smartpost pkg with a second dimension over 17" technically incurs a non-machinable surcharge, but eBay calculators can't deal with it. Yet the listing flow allows listings with that characteristic to go live. Then the live listing throws the invalid zip code error.
(the eBay label flow DOES allow labels to be generated with the appropriate non-machinable surcharge though)
That only happens when Smartpost is the only shipping option offered in the listing - if FedEx Ground or USPS services are also offered the Smartpost option just vanishes and no error is thrown.
If OP is serious about that item, OP might want to message seller and point them here or explain what follows. @24geep
Seller Workaround
If seller is actually using Smartpost as the only option, they can revise the listing and change the dims to avoid that calculator issue.
To get correct dimensional weight and correct rates charged, total volume of pkg must be kept the same*, but dims can be fudged to avoid the second dimension over 17" state. ie: if an 18x18x8 pkg, the volume is 18x18x18 = 2592 in³ Fake dimensions 20x16x8 don't trigger any non-machinable criteria, and yield a volume of 2560 in³
*which is close enough due to rounding up the dim weight to nearest pound obtained by dividing that Volume by the 250 eBay Smartpost dim weight divisor (all volumes greater than 2500 in³ and less or equal to 2750 in³ produce an 11 lb dimensional weight when divided by 250 and the result rounded up)
Note that a handling fee of $3.05 NEEDS to be added to the listing with those fake dimensions to get the calculated cost correct, and the real dimensions need to be used after the sale to create the label (because the shipped package with a label created using the right dimensions WILL be non-machinable, FedEx is going to charge that surcharge after the fact if label doesn't reflect actual dimensions, and then seller would be here claiming fraud, scam, ripoff by FedEx 
)
TLDNR; The seller probably has Smartpost as the only shipping option, pkg dimensions are greater that 17x17xN (probably and 18x18x8 box), and the eBay calcs are balking.
(Note: while verifying a few things here I found that there appear to be some really serious problems with FedEx Smartpost and dim weight and non-machinable in both the new and classic label flows, and some other restrictive changes. Will look into that.)
