I am not sure what is SAL. China Post does not have a service called SAL. Maybe your seller is talking about surface parcel.

Register Airmail is not expensive, actually it is very cheap. But if you want ship something over 2 kgs, you can not use register airmail. Only surface mail can be used to ship heavy item over 2kgs. Surface mail is very very very very slow. Depending on your destination , it might take 3 months to over half year to get the parcel.

The most reliable and affordable way to ship heavy item is to use EMS. I am not sure your country, but 4 kgs item should be less than US$90 by EMS if you are in Asia.

Now I understand what you are talking about. Your SAL actually means 空运水陆路包裹 in Chinese. This service is not quite commonly used by shipper.

Based on our statistics, 空运水陆路包裹 means some stage it uses air and some stages it uses ship or truck. The speed quite depends on your luck. In worst case, 空运水陆路包裹 can be as slow as surface parcel which takes half year. in best case, when most of its journey is by air, it will arrive France in about 2 to 4 weeks. You can not predict the arrival time at all.

If your goods is valuable, I still suggest you use EMS instead of 空运水陆路包裹. If you do want to use 空运水陆路包裹 to save money, you must pay by paypal which will force seller (shipper) to take the risk of parcel lost or serious delay. If you can not get it in 60 days, at least you can claim refund under paypal buyer protection policy.