How can I build this JSON file in R? - r

Question

Working in R, I am having difficulty building a JSON file that I would use in an API call.
The required format for the JSON file can be seen here:
https://developer.trimblemaps.com/restful-apis/routing/route-reports/post-route-reports/
The input to the exercise is a dataframe like so:
Shipper_Latitude <- c(1,2,3,4)
Shipper_Longitude <- c(1,2,3,4)
r_combine.NewShipperLat <- c(1,2,3,4)
r_combine.NewShipperLon <- c(1,2,3,4)
r4 <- data.frame(Shipper_Latitude,Shipper_Longitude,r_combine.NewShipperLat,r_combine.NewShipperLon)
My attempt at building the required JSON file is as follows:
# assemble lat and long for starting location:
tempfuna <- function(Lat,Lon) {list(Coords = list(Lat = Lat,Lon = Lon))}
df_jsona <- mapply(FUN = tempfuna,Lat = r4$Shipper_Latitude, Lon = r4$Shipper_Longitude)
df_jsona <- lapply(df_jsona, function(x) {list(Coords = x)})
# assemble lat and long for ending location:
tempfunb <- function(Lat,Lon) {list(Coords = list(Lat = Lat,Lon = Lon))}
df_jsonb <- mapply(FUN = tempfunb,Lat = r4$r_combine.NewShipperLat, Lon = r4$r_combine.NewShipperLon)
df_jsonb <- lapply(df_jsonb, function(x) {list(Coords = x)})
# assemble list of ReportRoutes:
tempfunc <- function(A,B) {list(ReportRoutes = list(Stops = list(A,B)))}
df_jsonc <- mapply(FUN = tempfunc,A = df_jsona, B = df_jsonb)
# create final list:
post_body <- list(ReportRoutes = df_jsonc)
I get an error when I use the resulting file in an API call.
I think the problem is that the list items in the ReportRoutes list are incorrectly named. For example, the first item is named “Coords.ReportRoutes” instead of [[1]]].
How can I rework the above to produce the required JSON file?