You can find details about check baggage allowances and fees here. Specifically: Southwest will accept the first two checked bags for free provided that the bags do not exceed 50 lbs and 62 inches (L+W+H). Overweight items from 50 to 100 pounds and oversized items in excess of 62 inches but not more than 80 inches will be accepted for a charge of $75 per item. Only one $75.00 charge applies if the piece is both oversized and overweight, regardless of the weight as long as it does not exceed 100 lbs. Excess baggage which is also overweight or oversized will be charged excess baggage plus the applicable oversize or overweight charge. Sporting Equipment includes windsurfing/surf boards, vaulting poles, non-motorized bicycles (62 inches or less and less than 50 pounds), life rafts, javelin, kayak. (emphasis mine) More detail available here as well. As far as the straps and securing the bags for checking go, I would ensure the straps are tightened as much as possible and that there is nothing hanging on the outside of the bags that might get caught up as the bag travels through the airport and in the plane. I did a quick Google search, and found this that might be helpful!
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