Changes

gistfile1.txt

// Index route, which I added route parameter to include ID
[HttpGet("estimate/{project_ID}")]
public IActionResult Index(int project_ID)
{
    // Any specific reason for the Session? Could
    // this be resolved from the current user / claims?
    if(HttpContext.Session.GetInt32("userID") == null)
    {
        return RedirectToAction("Index", "User");
    }
    // ViewBag.sessionUID = HttpContext.Session.GetInt32("userID");
    ViewBag.Materials = materialFactory.FindAll();
    ViewBag.EstimateMaterials = estimateFactory.ShowEstimateTable();
    ViewBag.EstimateTotals = estimateFactory.ShowEstimateTotals();

    // use .First() to store the first entry in the Enumerbal as this method
    // only returns one row from projects
    ViewBag.ProjectInfo = projectFactory.GetProjectInfo(project_ID).First();

    return View();
}

// form with asp-action
<form asp-action="AddProjectRentalRate" asp-controller="Estimate" method="post">
    <label>Rental Rate</label>
    <input type="number" step=".01" name="rental_rate">
    <input type="hidden" name="project_ID" value="@ViewBag.ProjectInfo.projectID">
    <input type="submit" name="submit">
</form>

// Upon form submission, error received: 
// No webpage was found for the web address: http://localhost:5000/estimate/1
// If I hit refresh, the page is loaded, but no changes have been made to the db.
[HttpPost("add_project_rental_rate")]
public IActionResult AddProjectRentalRate(Decimal rental_rate, int project_ID)
{
    if(HttpContext.Session.GetInt32("userID") == null)
    {
        return RedirectToAction("Index");
    }
    int UserID = (int)HttpContext.Session.GetInt32("userID");

    projectFactory.EditRentalRate(rental_rate, project_ID);
    return RedirectToAction("Index", new {project_ID = project_ID});
}