Relay protection when proper TVS at load cannot be implemented
I spent the last 15 years in the TVSS industry. You go by what UL and ISO standards call for and add labels to warn the customer that neglect or abuse can result in a voided warranty.

Having said all of that, for the ratings you gave, I would ship it with a 40mm MOV which has at least a 10 kA or 20 kA 275 VAC rating, across both N.O. and N.C. connections (2 MOV's total). It will hard-clamp at 420 VAC/DC or so. A very expensive solution is to use giant sidacs, and they have a sharp roll-off at the maximum allowed voltage. 275 VAC/DC means just that, but they can cost $40 USD each.

I would consider protection from the relay coil 'kickback' as well, but a diode or 20mm MOV will work ok.

There are thermally protected MOV's out there (TPMOV), but not for OTC sales.
Getting a third-party vendor for surge suppression would be very costly, as these products have a high labor cost.

I would try the 40mm 275 VAC/DC MOV's first. They can take 15 20 kA 'hits' (over 2 hours) and still pass the 1 mA test.

SNUBBERS: RC snubbers on AC circuits are not a good idea, as they allow a small amount of AC current to bypass the relay even if it is OFF. Not knowing if the end user is going to be using AC or DC means play it safe and avoid them. They cannot do what an MOV or Sidac does.

NOTES: The MOVs and Sidacs only see the kickback, or surge current which is a brief spike of 20 uS or so. They do not see the normal run current as they are in a very high resistance mode. Only the relay contacts see the 'run' current.

If 'inrush' current is welding the contacts, then you need a relay with a higher contact current rating. Add a 50% safety margin for long life. Use a moisture-tight relay if possible.

Plants that process citric products such as orange juice have an acidic atmosphere that corrodes steel and copper quickly.

Fusing: I should add that proper fusing for a 40mm MOV or large Sidac is a 30 amp 600 volt 200 kA rated fuse. They come in a box of ten for about $50 USD. they are not cheap fuses, as they are made with a hole-punched platinum strip, specific to blow on severe surges quickly, but tolerate motor start currents. You can use in-line locking fuse holders. These meet UL1449 editons 3 and 4 specs for fusing 40 mm MOVs. A link to the correct fuses:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-b/Bus_Ele_DS_1023_LP-CC.pdf