If you have a client to Sage 100, you can access Sage 100 through the BOI or Business Object Interface. Since you are looking for more of a Web API you would need to use something like e-Business Web Services which is an additional module but is very limited to Sales Order, Customer and Customer Contact. Other options are SDATA which will allow access to more things and is configurable. Development for Web Services and SDATA has not changed in many years. Sage has also introduced a connector for Microsoft 365 that opens access to Sage from the Microsoft ecosystem. It is limited in what it can do and is currently being developed but you have to be connected through O365.

Other options go into 3rd Party. There are a handful. InSynch (https://roi-consulting.com/) is a Sync tool not so much as an API interface but it is used for ecommerce Sites. Website Pipeline (CimCloud) offers an API (https://www.cimcloud.com/products/data-hubs.asp) access through their network. There is also Sage Data Exchange (http://www.sagedataexchange.com/) which is an API using JSON or XML and is customizable. There are others as well, but these are just a few. 

If you don't know much about Sage 100 then it would be wise to work with your Sage Partner or Consultant to help you find the right option to use.

If you have a client to Sage 100, you can access Sage 100 through the BOI or Business Object Interface. Since you are looking for more of a Web API you would need to use something like e-Business Web Services which is an additional module but is very limited to Sales Order, Customer and Customer Contact. Other options are SDATA which will allow access to more things and is configurable. Development for Web Services and SDATA has not changed in many years. Sage has also introduced a connector for Microsoft 365 that opens access to Sage from the Microsoft ecosystem. It is limited in what it can do and is currently being developed but you have to be connected through O365.

Other options go into 3rd Party. There are a handful. InSynch (https://roi-consulting.com/) is a Sync tool not so much as an API interface but it is used for ecommerce Sites. Website Pipeline (CimCloud) offers an API (https://www.cimcloud.com/products/data-hubs.asp) access through their network. There is also Sage Data Exchange (http://www.sagedataexchange.com/) which is an API using JSON or XML and is customizable. There are others as well, but these are just a few. 

If you don't know much about Sage 100 then it would be wise to work with your Sage Partner or Consultant to help you find the right option to use.