Rewolfe,
you should be able to use premade garlands, and adding your flowers to the garlands in the shop, transport it to site, and attach it to the birch arbor when you get there.
Do in in 3 to 5 pieces of garland . 1 pieceof graland across the topl 1 or two down each side.
Your dimensions help. But, How deep is the arbor (front to rear). this will determine how much material you trully need to do what you want to do. If the arbor is narrow, then you can get away with 1 garland across the top.
If the arbor will support it, you could also us a single cage or John Henry saddle, and design your materials out if that, coming across the top(from the middle) , with 2 cages (full or 1/2 to 1/3 size) coming down each side.
8' wide is a very large arbor!!!! Our around here are about 4 to 5 ft' wide maximum. Are you sure that your dimensions are correct???? I would want to make sure I have the correct dimensions before I work up a quote.
Based upon your dimensions given 7' across by 8" tall, you need, at minimum, 24 lineal feet of garland. (7' + 8' +8' +1' for fudge & cutting).
At $4/ft cost -- your price at retail for the garland alone would be $384.00.
(using a 4 x markup).
IF your dimensions are correct, then, the price you are quoting is reasonable if you are able to premake most of it at the shop, and attach on site.
If you are making this whole thing from scratch, on site, then your pricing is low. this would also be low if you have to do a lot of fussing & fluffing, etc on site. Labor on site costs lots of $$$$!
Cheryl
ps. i think part of what has you doubting everything is that the Labor is the Big unknown. You need to get a good handle on how much time it is going to take to install this thing, and how much time to make it in the shop. Pieces this big require 2 people in site to install. Don't forget to factor in the labor cost of the 2nd person.