First fiber run over 10km

Getting ready to connect a site that is roughly 13 km away with a direct fiber run. Our standard GBICs only run to 10 km so was looking into a new model. Our main line to all sites run Cisco switches (this will be connecting two Cisco 3850s) and have always used Cisco GBICs, but $1300 for a GLC-LH-SMD seems rough, especially when fs.com has theirs for $14 https://www.fs.com/products/20358.html. We have never used FS, but I hear great things from on reddit all the time. Thinking about grabbing some just to try out, but wanted to check if I am just missing something completely between these or if anyone has had experience with them vs the Cisco branded ones. Extra stress point, only one side of these will be in a controlled environment where the new location is exposed to temps sub freezing and over 120^o F. We have had Cisco 2960s and Cisco GBICs in these conditions for over 15 years now without a failure.

Also, I have never come across this statement before on a GBIC: A 5 dB optical attenuator should be inserted between the fiber-optic cable and the receiving port on the SFP at each end of the link for back-to-back connectivity. Is this a standard statement for any run length with these transceivers, and if so, why not just make the laser 5 dB less powerful to begin with. I assume since i would only be using 13 km of a 40 km possible run the light would be a bit overbearing, but my lack of knowledge in this area does not want me to just go for it without questioning first.